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IMMUNE SYSTEM   Leave a comment

DEFINITION: the system that protects the body from disease by producing antibodies.

All living organisms are continuously exposed to substances that are capable of causing them harm. Most organisms protect themselves against such substances in more than one way with physical barriers, for example, or with chemicals that repel or kill invaders. Animals with backbones, called vertebrates, have these types of general protective mechanisms, but they also have a more advanced protective system called the immune system. The immune system is a complex network of organs containing cells that recognize foreign substances in the body and destroy them. It protects vertebrates against pathogens, or infectious agents, such as viruses, bacteria, fungi, and other parasites. The human immune system is the most complex and is the focus of this article.

Although there are many potentially harmful pathogens, no pathogen can invade or attack all organisms because a pathogen’s ability to cause harm requires a susceptible victim, and not all organisms are susceptible to the same pathogens. For instance, the virus that causes AIDS in humans does not infect animals such as dogs, cats, and mice. Similarly, humans are not susceptible to the viruses that cause canine distemper, feline leukaemia, and mouse pox.

Two Kinds of Immunity

All animals possess a primitive system of defence against the pathogens to which they are susceptible. This defence is called innate, or natural, immunity and includes two parts. One part, called humoral innate immunity, involves a variety of substances found in the humors, or body fluids. These substances interfere with the growth of pathogens or clump them together so that they can be eliminated from the body. The other part, called cellular innate immunity, is carried out by cells called phagocytes that ingest and degrade, or “eat,” pathogens and by so-called natural killer cells that destroy certain cancerous cells. Innate immunity is non-specific that is, it is not directed against specific invaders but against any pathogens that enter the body.

Antigen, a substance in blood that causes production of antibodies against itself.

Only vertebrates have an additional and more sophisticated system of defence mechanisms, called adaptive immunity, that can recognize and destroy specific substances. The defensive reaction of the adaptive immune system is called the immune response. Any substance capable of generating such a response is called an antigen, or immunogen. Antigens are not the foreign micro-organisms and tissues themselves; they are substances such as toxins or enzymes in the micro-organisms or tissues that the immune system considers foreign. Immune responses are normally directed against the antigen that provoked them and are said to be antigen-specific. Specificity is one of the two properties that distinguish adaptive immunity from innate immunity. The other is called immunologic memory. Immunologic memory is the ability of the adaptive immune system to mount a stronger and more effective immune response against an antigen after its first encounter with that antigen, leaving the organism better able to resist it in the future.

Adaptive immunity works with innate immunity to provide vertebrates with a heightened resistance to micro-organisms, parasites, and other intruders that could harm them. However, adaptive immunity is also responsible for allergic reactions and for the rejection of transplanted tissue, which it may mistake for a harmful foreign invader.

Lymphocytes Heart of the Immune System

Antigen, a substance in blood that causes production of antibodies against itself.

Lymphocytes a class of white blood cells are the principal active components of the adaptive immune system. The other components are antigen-presenting cells, which trap antigens and bring them to the attention of lymphocytes so that they can mount their attack.

How lymphocytes recognize antigens. A lymphocyte is different from all other cells in the body because it has about 100,000 identical receptors on its cellular membrane that enable it to recognize one specific antigen. The receptors are proteins containing grooves that fit into patterns formed by the atoms of the antigen molecule somewhat like a key fitting into a lock so that the lymphocyte can bind to the antigen. There are more than 10 million different types of grooves in the lymphocytes of the human immune system.

When an antigen invades the body, normally only those lymphocytes with receptors that fit the contours of that particular antigen take part in the immune response. When they do, so-called daughter cells are generated that have receptors identical to those found on the original lymphocytes. The result is a family of lymphocytes, called a lymphocyte clone, with identical antigen-specific receptors.

A clone continues to grow after lymphocytes first encounter an antigen so that, if the same type of antigen invades the body a second time, there will be many more lymphocytes specific for that antigen ready to meet the invader. This is a crucial component of immunologic memory.

How lymphocytes are made. Like all blood cells, lymphocytes are made from stem cells in the bone marrow. (In foetuses, or unborn offspring, lymphocytes are made in the liver.) Lymphocytes then undergo a second stage of development, or processing, in which they acquire their antigen-specific receptors. By chance, some lymphocytes are created with receptors that happen to be specific to normal, healthy components of the body. Fortunately, a healthy immune system purges itself of these lymphocytes, leaving only lymphocytes that ignore normal body components but react to foreign intruders. If this purging process is not completely successful, the result is an autoimmune (literally “self-immune”) disease in which the immune system attacks normal components of the body as though they were foreign antigens, destroying healthy molecules, cells, or tissues.

Some lymphocytes are processed in the bone marrow and then migrate to other areas of the body specifically the lymphoid organs. These lymphocytes are called B lymphocytes, or B cells (for bone-marrow-derived cells). Other lymphocytes move from the bone marrow and are processed in the thymus, a pyramid-shaped lymphoid organ located immediately beneath the breastbone at the level of the heart. These lymphocytes are called T lymphocytes, or T cells (for thymus-derived cells).

These two types of lymphocytes B cells and T cells play different roles in the immune response, though they may act together and influence one another’s functions. The part of the immune response that involves B cells is often called humoral immunity because it takes place in the body fluids. The part involving T cells is called cellular immunity because it takes place directly between the T cells and the antigens. This distinction is misleading, however, because, strictly speaking, all adaptive immune responses are cellular that is, they are all initiated by cells (the lymphocytes) reacting to antigens.

Antibody, the protective substance produced in body fluids in response to exposure to foreign antigen in blood.

B cells may initiate an immune response, but the triggering antigens are actually eliminated by soluble products that the B cells release into the blood and other body fluids. These products are called antibodies and belong to a special group of blood proteins called immunoglobulins. When a B cell is stimulated by an antigen that it encounters in the body fluids, it transforms, with the aid of a type of T cell called a helper T cell, into a larger cell called a blast cell. The blast cell begins to divide rapidly, forming a clone of identical cells.

Some of these transform further into plasma cells in essence, antibody-producing factories. These plasma cells produce a single type of antigen-specific antibody at a rate of about 2,000 antibodies per second. The antibodies then circulate through the body fluids, attacking the triggering antigen.

Antibodies attack antigens by binding to them. Some antibodies attach themselves to invading micro-organisms and render them immobile or prevent them from penetrating body cells. In other cases, the antibodies act together with a group of blood proteins, collectively called the complement system, that consists of at least 30 different components. In such cases, antibodies coat the antigen and make it subject to a chemical chain reaction with the complement proteins. The complement reaction either can cause the invader to burst or can attract scavenger cells that “eat” the invader.

Not all of the cells from the clone formed from the original B cell transform into antibody-producing plasma cells; some serve as so-called memory cells. These closely resemble the original B cell, but they can respond more quickly to a second invasion by the same antigen than can the original cell.

T cells. There are two major classes of T cells produced in the thymus: helper T cells and cytotoxic, or killer, T cells. Helper T cells secrete molecules called interleukins (abbreviated IL) that promote the growth of both B and T cells. The interleukins that are secreted by lymphocytes are also called lymphokines. The interleukins that are secreted by other kinds of blood cells called monocytes and macrophages are called monokines. Some ten different interleukins are known: IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, interferon, lymphotoxin, and tumour necrosis factor. Each interleukin has complex biological effects.

Cytotoxic T cells destroy cells infected with viruses and other pathogens and may also destroy cancerous cells. Cytotoxic T cells are also called suppressor lymphocytes because they regulate immune responses by suppressing the function of helper cells so that the immune system is active only when necessary.

The receptors of T cells are different from those of B cells because they are “trained” to recognize fragments of antigens that have been combined with a set of molecules found on the surfaces of all the body’s cells. These molecules are called MHC molecules (for major histocompatibility complex). As T cells circulate through the body, they scan the surfaces of body cells for the presence of foreign antigens that have been picked up by the MHC molecules. This function is sometimes called immune surveillance.

Immune Response

When an antigen enters the body, it may be partly neutralized by components of the innate immune system. It may be attacked by phagocytes or by preformed antibodies that act together with the complement system. Often, however, the lymphocytes of the adaptive immune system are brought into play.

The human immune system contains approximately 1 trillion T cells and 1 trillion B cells, located in the lymphoid organs and in the blood, plus approximately 10 billion antigen-presenting cells located in the lymphoid organs. To maximize the chances of encountering antigens wherever they may invade the body, lymphocytes continually circulate between the blood and certain lymphoid tissues. A given lymphocyte spends an average of 30 minutes per day in the blood and recirculates about 50 times per day between the blood and lymphoid tissues.

If lymphocytes encounter an antigen trapped by the antigen-presenting cells of the lymphoid organs, lymphocytes with receptors specific to that antigen stop their migration and settle to mount an immune response locally. As these lymphocytes accumulate in the affected lymphoid tissue, the tissue often becomes enlarged for example, the lymph nodes in the groin become enlarged if there is an infection in the thigh area.

Antigen-presenting cells degrade antigens and often eliminate them without the help of lymphocytes. If there are too many antigens for them to handle alone, however, the antigen-presenting cells secrete IL-1 and display fragments of the antigens (combined with MHC molecules) to alert the helper T cells. The IL-1 facilitates the responsiveness of T and B cells to antigens and, if released in large amounts (as it is in the course of infections), can also cause fever and drowsiness. Helper T cells that encounter IL-1 and fragments of antigens transform into cells called lymphoblasts, which then secrete a variety of interleukins that are essential to the success of the immune response. The IL-2 produced by helper T cells promotes the growth of cytotoxic T cells, which may be necessary to destroy tumorous cells or cells infected with viruses. The IL-3 increases the production of blood cells in the bone marrow and thus helps to maintain an adequate supply of the lymphocytes and lymphocyte products necessary to fight infections. Helper T cells also secrete interleukins that act on B cells, stimulating them to divide and to transform into antibody-secreting plasma cells. The antibodies then perform their part of the immune function.

The process of inducing an immune response is called immunization. It may be either natural through infection by a pathogen or artificial through the use of serums or vaccines. The heightened resistance acquired when the body responds to infection is called active immunity. Passive immunity results when the antibodies from an actively immunized individual are transferred to a second, non immune subject. Active immunization, whether natural or artificial, is longer-lasting than is passive immunization because it takes advantage of immunologic memory.

Monoclonal Antibodies

Scientists can now produce antibody-secreting cells in the laboratory by a method known as the hybridoma technique. Hybridomas are hybrid cells made by fusing a cancerous, or rapidly reproducing, plasma cell and a normal antibody-producing plasma cell obtained from an animal immunized with a particular antigen. The hybridoma cell can produce large amounts of identical antibodies called monoclonal, or hybridoma, antibodies which have widespread applications in medicine and biology.

Assisted by Jose Quintans, Professor of Pathology and Immunology, University of Chicago, and recipient of the Quantrell Award for excellence in undergraduate teaching.

HUMAN DISEASES (Part 1 of 7)   Leave a comment

A disease is a condition that impairs the proper function of the body or of one of its parts. Every living thing, both plants and animals, can succumb to disease. People, for example, are often infected by tiny bacteria, but bacteria, in turn, can be infected by even more minute viruses.

Hundreds of different diseases exist. Each has its own particular set of symptoms and signs, clues that enable a physician to diagnose the problem. A symptom is something a patient can detect, such as fever, bleeding, or pain. A sign is something a doctor can detect, such as a swollen blood vessel or an enlarged internal body organ.

Every disease has a cause, although the causes of some remain to be discovered. Every disease also displays a cycle of onset, or beginning, course, or time span of affliction, and end, when it disappears or it partially disables or kills its victim.

Endemic disease (also called childhood disease), disease continually prevalent in a region.

An epidemic disease is one that strikes many persons in a community. When it strikes the same region year after year it is an endemic disease.

An acute disease has a quick onset and runs a short course. An acute heart attack, for example, often hits without warning and can be quickly fatal. A chronic disease has a slow onset and runs a sometimes years-long course. The gradual onset and long course of rheumatic fever makes it a chronic ailment.

How Germs Invade the Body

Humans live in a world where many other living things compete for food and places to breed. The pathogenic organisms, or pathogens, often broadly called germs, that cause many diseases are able to invade the human body and use its cells and fluids for their own needs. Ordinarily, the body’s defence system can ward off these invaders.

Pathogenic organisms can enter the body in various ways. Some such as those that cause the common cold, pneumonia, and tuberculosis are breathed in. Others such as those that cause venereal diseases enter through sexual contact of human bodies. Still others such as those that cause bacillary dysentery, cholera, and typhoid fever get in the body through contaminated food, water, or utensils.

Insects can spread disease by acting as vectors, or carriers. Flies can carry germs from human waste or other tainted materials to food and beverages. Germs may also enter the body through the bite of a mosquito, louse, or other insect vector.

Kinds of Disease

Infectious, or communicable, diseases are those that can be passed between persons such as by means of airborne droplets from a cough or sneeze. Tiny organisms such as bacteria and fungi can produce infectious diseases. So can viruses. So can tiny worms. Whatever the causative agent might be, it survives in the person it infects and is passed on to another. Or, its eggs are passed on. Sometimes, a disease-producing organism gets into a person who shows no symptoms of the disease. The asymptomatic carrier can then pass the disease on to someone else without even knowing he has it.

Non-infectious, or non-communicable, diseases are caused by malfunctions of the body. These include organ or tissue degeneration, erratic cell growth, and faulty blood formation and flow. Also included are disturbances of the stomach and intestine, the endocrine system, and the urinary and reproductive systems. Some diseases can be caused by diet deficiencies, lapses in the body’s defence system, or a poorly operating nervous system.

Disability and illnesses can also be provoked by psychological and social factors. These ailments include drug addiction, obesity, malnutrition, and pollution-caused health problems.

Furthermore, a thousand or more inheritable birth defects result from alternations in gene patterns. Since tiny genes are responsible for producing the many chemicals needed by the body, missing or improperly operating genes can seriously impair health. Genetic disorders that affect body chemistry are called inborn errors of metabolism. Some forms of mental retardation are hereditary.


Mucous membrane (or mucosa), membrane that secretes mucus and lines the mouth, nose, throat, windpipe, lungs, eyelids, and the alimentary canal.

As a first line of defence, a healthy body has a number of physical barriers against infection. The skin and mucous membranes covering the body or lining its openings offer considerable resistance to invasion by bacteria and other infectious organisms. If these physical barriers are injured or burned, infection resistance drops. In minor cases, only boils or pimples may develop. In major cases, however, large areas of the body might become infected.

Cilia (plural of cilium), hairlike, vibratory appendages found in some plants and animals.

Breathing passages are especially vulnerable to infection. Fortunately, they are lined with mucus-secreting cells that trap tiny organisms and dust particles. Also, minute hairs called cilia line the breathing passages, wave like a field of wheat, and gently sweep matter out of the respiratory tract. In addition, foreign matter in the breathing passages can often be ejected by nose blowing, coughing, sneezing, and throat clearing. Unfortunately, repeated infection, smoking, and the repeated use of strong chemicals (including alcohol and drugs) can damage the respiratory passageways and make them more susceptible to infection.

Scavenger cells are present too in the walls of the bronchi, the branched air tubes to the lungs. Foreign matter reaching the bronchi after evading the other defences can be “eaten” by the scavengers and disposed of in the lymph glands of the lungs.

Many potential invaders cannot stand body temperature (98.6° F or 37° C). Even those that thrive at that temperature may be destroyed when the body assumes higher, fever temperatures.

Wax in the outer ear canals and tears from eye ducts can slow the growth of some bacteria. And stomach acid can destroy certain swallowed germs.

Lymph, a colourless liquid exuded through capillaries to nourish tissues of the body.

The body’s second line of defence is in the blood and lymph. Certain white blood cells flock to infected areas and try to localize the infection by forming pus-filled abscesses. Unless the abscess breaks and allows the pus to drain, the infection is likely to spread. When this happens, the infection is first blocked by local lymph glands. For example, an infection in the hand travels up the arm, producing red streaks and swollen, tender lymph glands in the armpit. Unless the infection is brought under control, it will result in blood poisoning.

Scavenger cells, or phagocytes, are located at various sites to minimize infection. One type in the spleen and liver keeps the blood clean. Others in such high-risk areas as the walls of the bronchi and the intestines remove certain bacteria and shattered cells.

How We Become Immune to Disease

The body has a special way of handling infection. It has a system that fends off the first traces of an infectious substance and then, through a “memory,” gives the body a long-lasting immunity against future attacks by the same kind of invader.

Antigen, a substance in blood that causes production of antibodies against itself.

Antibody, the protective substance produced in body fluids in response to exposure to foreign antigen in blood.

Many substances could harm the body if they ever entered it. These substances, or antigens, range from bacteria and pollen to a transplanted organ (viewed by the body as an invader). To fight them the body makes special chemicals known as antibodies.

Antibodies are a class of proteins called immuno-globulins. Each antibody is made of a heavy chain of chemical subunits, or amino acids, and a light chain of them. The light chain has special sites where the amino acids can link with their complements on the antigen molecule. When an antibody hooks up with an antigen, it often puts the antigen out of action by inactivating or covering a key portion of the harmful substance. In some cases, through the process of opsonization, antibodies “butter” the surface of some antigens and make them “tastier” to phagocytes, which engulf the antigens. Sometimes an antibody hooks to a bacterial antigen but needs an intermediate, or complement, to actually destroy the bacterium. As the antibody-antigen complex circulates in the blood, the complex “fixes” complement to it. In turn, the complement causes powerful enzymes to eat through the bacterial cell wall and make the organism burst.

There are several kinds of immuno-globulins IgM, the largest; IgG, the most plentiful and versatile; and IgA, the next most plentiful and specially adapted to work in areas where body secretions could damage other antibodies. Other immuno-globulins are tied in with allergic reactions. IgM is made at the first signs of an antigen. It is later supplanted by the more effective IgG.

When infection first strikes, the immunity system does not seem to be working. During the first day or so, antibodies against the infection cannot be found in the blood. But this is only because the basic cells involved in antibody production have been triggered by the presence of antigen to multiply themselves. The antibody level starts to rise on about the second day of infection and then zooms upward. By the fifth day the antibody level has risen a thousandfold.

The first antibodies, the large IgM type, are not the best qualified to fight a wide range of antigens, but they are particularly effective against bacteria. The more versatile IgG is circulating in the blood on about the fourth day of infection. Its production is stimulated by the rising level of IgM in the blood. At this time, IgM production drops off and the immunity system concentrates on making IgG. The IgG type of antibody sticks well to antigens and eventually covers them so that the antigens can no longer stimulate the immune response and IgG production is switched off. This is an example of negative feedback control.

Antibody Production

Thymus, organ, located behind the breastbone and above the heart; participates in the production of white blood cells or lymphocytes; because it attains maximum size at puberty and becomes smaller in adults, researchers feel it may be an endocrine gland that affects growth and sexual maturation.

Antibodies are made by two kinds of cells plasma cells and a class of white blood cells, lymphocytes. Plasma cells actually originate from lymphocytes and are found throughout the lymphatic tissue. Lymphocytes stem from cells in the blood-forming sections of bone marrow. When the bone-marrow cells circulate to the thymus, a lymphatic structure in the chest, they receive “orders” to become lymphocytes and make antibodies. Most lymphocytes last for only a few hours, but a few wander through the blood and body tissues for years. These lymphocytes are responsible for “remembering” old antigens and for inducing the immunity system to produce antibodies against those or similar antigens if they ever again enter the body.

When people develop antibodies against a disease by the action of their own immunity system, they have active immunity. When they are given someone else’s antibodies, however, they just have passive immunity to a disease.

Passive immunity is only temporary. Some people may also get temporary relief from a disease through injections of serum containing gamma globulin, a portion of the blood rich in antibodies.

Without protective antibodies, we could die of the first disease that struck us. This would be true, too, of newborn babies, except that they receive passive immunity from their mothers. During her lifetime, a mother accumulates a wide variety of antibodies against a host of diseases. Enough of them are passed to the developing baby in her womb to give it a temporary immunity to many diseases during the early months of its life, until it can develop its own set of antibodies.

HUMAN DISEASES (Part 2 of 7)   Leave a comment

What Happens When Immunity Backfires

Paradoxically, a person’s immunity system can backfire and develop auto-antibodies against his own body tissue. In many diseases of unknown cause, doctors have found many unusual antibodies in the blood serum of patients.

Rheumatoid arthritis (RA), chronic disease of the connective tissue, causing painful sensations in joints and muscles.

Doctors think the patients become sensitive to something made by their own bodies. Only a slight change in certain proteins in normal body tissue is necessary for them to become antigens. Most diseases marked by the production of auto-antibodies cannot be traced to infection or drug allergy. In rheumatoid arthritis, for example, the rheumatoid factor is the presence of auto-antibodies in the victim’s blood. These auto-antibodies may stick to the membranes lining the bone joints and cause a reaction that destroys tissue in the joints. In other disorders associated with reversed immunity, auto-antibodies strike red blood cells, tissues surrounding small blood vessels, or other target areas. Ulcerative colitis, a disorder marked by an inflamed portion of the intestine, often with ulcers, is also believed to be an autoimmune disease.

In some cases, lymphocyte defects or discrepancies in antibody production lead to an immune deficiency. The victim is then helpless against recurring infections. A simple head cold can soon become pneumonia. Antibiotics or serums with antibody-rich gamma globulin offer temporary relief in such cases.

1796: Inoculation against disease. The simple medical procedure known as vaccination first came into use in about 1713 as a means of preventing smallpox. Such inoculation sometimes proved dangerous, because individuals sometimes caught a severe case of the disease instead of a mild one. This problem was solved by Edward Jenner, a British physician, in 1796. He realized that individuals inoculated with the much milder cowpox virus became immune to smallpox. Jenner tested his theory in May 1796.

This kind of inoculation earned the name vaccine, from the Latin word vaccinus, meaning “from cows.” Since Jenner’s day vaccines have been developed to fight polio, diphtheria, whooping cough, measles, typhoid fever, cholera, tetanus, and other diseases.

1928: Penicillin. In 1928 the Scottish bacteriologist Alexander Fleming was doing research on the Staphyloccus bacteria when he noticed that a growth of mould Penicillium notatium was contaminating the culture. There was no bacteria where the mould was present. Following up on this fact, Fleming found there was something in the mould that prevented bacterial growth. He named this substance penicillin.

By continued experiment Fleming learned that penicillin is capable of killing many common disease-causing bacteria. His discovery proved to be one of the first and most useful antibiotics used in medicine today. By 1940 penicillin had been turned into an injectable medicine. Its use grew dramatically during World War II as an infection-preventing agent.


With the advent of drug therapy in the 20th century, doctors began to use lifesaving drugs to fight disease. The clinical use of sulphanilamide, the predecessor of sulphur drugs, in the 1930s and the mass production of penicillin, the first antibiotic, in the 1940s gave physicians extremely powerful tools with which to fight infection. A disease-fighting drug never acts by itself. It always works in conjunction with the body’s immunity system. Vaccines have also become available for the prevention of certain diseases.

How Certain Drugs Quell Infection

Such antibiotics as penicillin, streptomycin, and tetracycline are very effective against bacterial infections. The name “antibiotic” comes from antibiosis, or the use of substances made by one living thing to kill another. Antibiotics are made by bacteria and moulds that are specially cultured in commercial drug laboratories.

Antibiotics kill bacteria and other disease organisms in various ways. Some destroy the cell walls of bacteria. Others interfere with bacterial multiplication or fatally alter the way bacteria make vital proteins. Still others mix up the genetic blueprints of the bacteria.

Ordinarily, an antibiotic tricks bacteria into using the antibiotic’s chemicals instead of closely related ones that the organisms really need for making the key enzymes required for their growth and reproduction. With the antibiotic assimilated into their systems instead of the vital chemicals, an essential activity or structure of the pathogens is lacking and they die.

Sulphur drugs act in a somewhat similar but less effective way. Weakened but not killed by the sulphur drugs, the pathogens fall easy prey to the body’s scavenger cells. Drugs are also available against parasitic worms, infectious amoeba, and other pathogenic organisms.

Antibiotics are not very effective against viruses because the drugs cannot get into the body cells where viruses hide and multiply. However, the body produces a protein called interferon that inhibits viral reproduction.

A drug is sometimes recognized by the body’s immunity system as an antigen. It then triggers a severe reaction. In some cases, a person can suffer anaphylaxis, or extreme sensitivity, to penicillin after repeated injections. Without quick medical aid, severe cases of anaphylactic shock can be fatal.

How Bacteria Become Drug Resistant

Once in every several hundred million cell divisions a mutation makes a bacterium immune to an antibiotic drug. The mutation alters the bacterium’s genetic code and thus its ability to use certain chemicals for its life activities. Mutations can be caused by the radiations from outer space that stream into the Earth’s atmosphere, as well as by some atmospheric chemicals. As a result of the mutation, all bacteria that stem from the immune germ will be resistant to the drug unless any of them undergoes a mutation that makes the strain susceptible again. Hence, whenever a new antibiotic is developed, there will be a chance that bacteria will develop an immunity against it. But because mutations are fairly rare, doctors have a good chance of fighting a bacterial disease with the new drug before future strains become resistant.

Some members of a bacterial strain are resistant to certain drugs naturally. In the course of time they can eventually become selected through evolutionary forces to become the dominant drug-resistant forms of a pathogenic strain.

More importantly, some bacteria can pass on their drug resistance to bacteria of another strain by “infection.” Since the passing of resistance factors does not depend upon the lengthy process of mutation, it poses a much greater problem of drug immunity. As a consequence, doctors often must prescribe more than one antibiotic to fight certain diseases in the hope that this will slow bacterial resistance.

Use of Vaccines and Hormones

A person can become artificially immune to some diseases by means of a vaccine. Vaccines contain antigens that stimulate the production of protective antibodies. Immunity to smallpox, polio, measles, rabies, and certain other diseases, is induced by injecting a person with vaccines containing living but attenuated, or weakened, disease organisms.

A vaccine containing only dead organisms protects against typhoid fever and whooping cough, as well as against measles and polio. Vaccines containing toxins, or poisons, are used to prevent diphtheria and tetanus. When injected into a person, they trigger the production of special antibodies called antitoxins.

Some body disorders are caused by too much or too little hormone production. Hormones are body chemicals that influence many vital biochemical reactions. When someone suffers a hormone deficiency, a doctor usually can treat the deficiency with hormone shots.

1347: Black Death. The plague is one of the most devastating diseases that has ever afflicted mankind. It is a highly contagious fever caused by the bacillus Yersinia pestis, which is carried by fleas that infest rats.

The plague, commonly called bubonic plague or the Black Death, has been known since ancient times, but the best documented instance was its deadly appearance in Europe in 1347. It raged throughout all of Europe, killing at least one-fourth of the population probably 25 million people. Without understanding how it is spread, people had no defence against the disease. Poor sanitary conditions and the disruption of war only worsened the epidemic.

In Europe the epidemic started in Sicily and was spread by shipboard rats to other Mediterranean ports. It moved to North Africa, Italy, Spain, England, and France. By 1349 it made its way to Austria, Germany, Switzerland, and the Low Countries. By 1350 it reached Scandinavia and the Baltic states.

In general, the population of Europe did not recover to its size before the plague until the 16th century, and some towns never recovered. The immediate results of the plague a general collapse of economies, a breakdown of class relationships, and a halt to wartime hostilities forced a massive restructuring of society. It has had a lasting impact on art, literature, and religious thought.


Infectious diseases can be transmitted in many ways. They can be spread in droplets through the air when infected persons sneeze or cough. Whoever inhales the droplets can then become infected. Some diseases can be passed through contaminated eating or drinking utensils. Some can be spread through sexual activity. Others can even be spread in the course of medical or surgical treatment, or through the use of dirty injection equipment, especially by drug abusers.

Cold (also called common cold, or coryza), illness, acute inflammation of upper respiratory tract.

Once an infectious organism gains a foothold in the body, it begins to thrive and multiply. Its success is slow or fast, depending upon the nature of the pathogen. The symptoms of the common cold, for example, appear within a few days of infection. However, the symptoms of kuru, an uncommon disease of the nervous system, often appear three years or longer after infection.

Incubation period, length of time before the symptoms of a disease appear.

Every infectious disease has an incubation period. This is the length of time between the pathogen’s gaining a foothold in the body and the appearance of the first symptoms of the disease.

Several factors also determine whether a person will become the victim of a disease after being infected. The number of invading germs the dose of the infection influences the outbreak of disease. So does the virulence of the pathogens; that is, their power to do harm. In addition, the condition of the body’s immunological defences also affects the probability of catching a disease.

Contagious Disease

A great many infectious diseases are contagious; that is, they can easily be passed between people. To acquire certain contagious diseases someone need only be in the presence of someone with it, or come in contact with an infected part of the body, or eat or drink from contaminated utensils.

Someone can be a carrier of a contagious disease in several ways. He can be an asymptomatic carrier, or have a disease without ever developing its symptoms. He can be an incubationary carrier and pass on the pathogens at any time during the “silent” incubation period. He can be a convalescent carrier and transmit some of the infectious organisms remaining in the body even after recovery. Of course, anyone suffering the frank symptoms of a contagious disease can pass it on to others while the disease is running its course.


Disease of the heart or of the blood vessels, called cardiovascular disease, is the leading cause of death in the United States and Canada. It claims more than a million lives each year in the United States; more than 70,000 each year in Canada.

The heart is a muscular pump. When its own tissue or blood vessels become diseased, serious and sometimes fatal harm can follow.

Coronary Artery Disease

Disease of the coronary arteries that supply oxygen and nutrients to the heart is the most common heart ailment. Coronary artery disease accounts for more than a third of all deaths among males in the United States between the ages of 35 and 55. It also strikes many women past the age of 50. Hypertension (high blood pressure), overweight, cigarette smoking, diabetes mellitus, excess cholesterol, triglycerides and other fats in the blood, and probably lack of regular exercise contribute to the chance of getting coronary artery disease.

Coronary artery disease is characterized by an atheroma, a fatty deposit of cholesterol beneath the inner lining of the artery. The atheroma obstructs the passage of blood, thereby reducing the flow of nourishing blood to the heart muscle. It also sets up conditions for a blood clot in the coronary artery. Atheroma formation seems to run in families. Eating foods rich in saturated animal fat and cholesterol is also thought to contribute to atheroma formation.

Many persons with coronary artery disease do not experience symptoms. If the obstruction is bad enough, however, it may cause angina pectoris, myocardial infarction, or heart enlargement and failure.

Angina pectoris, brief paroxysm of severe chest pain with feeling of suffocation.

Angina pectoris is a chest pain that feels like something is squeezing or pressing the chest during periods of physical exertion. It takes place when the heart’s oxygen needs cannot be met because of a blocked coronary artery. Rest will relieve the pain. Some persons have angina pectoris for years and still live active lives.

Myocardial infarction is commonly called heart attack. Tissue death that results from a lack of blood is called infarction. When the coronary artery becomes so obstructed that the myocardium, or heart muscle, does not receive oxygen, it dies.

Heart attack (also called myocardial infarction, or coronary occlusion), an acute episode of heart disease.

Once, it was believed that a blood clot occluded the coronary artery and caused the infarction. This is why a heart attack is sometimes called a coronary occlusion. However, it now appears that most clots form in the artery after the infarction.

The first few hours after a heart attack are the most critical because abnormal heart rhythms may develop. Ventricular fibrillation is the most dangerous. The ventricles of the heart contract so fast that the pumping action is baulked Death follows in three or four minutes. Heart attack patients are usually treated in the coronary care unit of a hospital for a few days to enable electronic monitoring of the heart rate and rhythm.

Heart failure, condition that develops when repeated heart attacks occur.

Heart failure can occur when repeated heart attacks put too much strain on the remaining healthy heart muscle. As attacks destroy more and more heart muscle, the remaining muscle hypertrophies, or enlarges, to maintain effective pumping. Pressure builds up in a weakened heart, however, and causes fluid backup in the lungs. As a result, the heart output cannot keep pace with the body’s oxygen demands.

HUMAN DISEASES (Part 3 of 7)   Leave a comment

Heart Rhythm and Pacemakers

A node of special cells in the heart controls its rhythm by regularly producing energizing electrical signals. Sometimes, abnormal signals cause extra heart beats, or tachycardia.

At other times, especially in older persons, the signals might not be conducted too well through the heart, thus slowing it. When a person’s heart rate drops below 40 beats a minute, he usually feels faint and cannot function well. In that case, he often can be fitted with an artificially powered heart pacemaker.

Diagnosis and Treatment of Heart Trouble

A doctor carefully questions and examines anyone suspected of heart trouble for evidence of pain, fatigue, abnormal heartbeat, and so on. He listens to the heart and lungs with a stethoscope. Sometimes, a heart murmur, a rushing noise heard through the stethoscope, provides a clue to a heart problem. A faint murmur can be normal, but a loud one usually indicates a diseased heart valve or other trouble. A chest X ray is usually taken to get a picture of the heart and lungs. An electrocardiogram reveals the electrical activity of a patient’s heart.

A doctor can also rely on cardiac catheterization and angiography to diagnose heart disease. Cardiac catheterization involves slipping a catheter, a long tube, through veins into the heart to learn such things as how much blood the heart is pumping, whether its valves are damaged, and whether it is contracting as it should. Angiography involves injecting dye through a catheter into the heart so that subsequent X rays will reveal the internal anatomy of the heart and the blood flow through it.

Rheumatic fever, inflammatory disease probably caused by bacterial infection; damages connective tissue of the heart and joints.

Rheumatic Heart Disease

Rheumatic heart disease has both an acute form and a chronic form. The acute form, rheumatic fever, inflames joints and heart muscle. The joints always recover, but if the condition becomes chronic the heart valves may eventually become scarred. Rheumatic fever most often affects the mitral, or bicuspid, valve of the heart and produces a blockage called mitral stenosis.

Rheumatic fever is a health problem in many of the world’s developing nations. It is caused by an unusual body response to an infectious sore throat sparked by the bacterium beta haemolytic streptococcus. Uniquely, the bacterial cell wall and the human heart muscle have a protein in common. A person with a “strep” throat develops antibodies against the bacterial protein. However, the antibodies may also attack that person’s own heart muscle, damaging it over the years. Penicillin and other antibiotics treat strep throat and can prevent heart damage. In severe cases after many years, however, surgery might be needed to repair or even replace a damaged heart valve.

Hypertensive Heart Disease

Hypertension, or high blood pressure, is a fairly common disorder. Ordinarily, the heart creates sufficient pressure to send blood throughout the body. However, sometimes resistance to blood flow from the arteries is high and the blood pressure rises above normal. Because the heart must then work harder to maintain the higher pressure, it enlarges.

Blood pressure is maintained by means of a complex interaction between the heart, the nervous system, and a kidney hormone called renin. Some persons with hypertension have too much renin in their blood. High blood pressure increases the wear and tear on blood vessels. It also can cause heart failure, strokes, and kidney disorders. When discovered soon enough, it can be treated with drugs.

Other Kinds of Heart Disease

Sometimes the heart does not develop properly and a child can be born with a serious congenital heart disease. Heart valves might be too narrow or missing altogether, or the septum, a wall separating the heart chambers, might be incomplete. As a result, a hole exists between the heart chambers. Such congenital heart diseases can be discovered by means of cardiac catheterization and angiography and often can be corrected by a heart surgeon.

Some substances are dangerous to the heart. For example, diphtheria bacteria produce a toxin that damages the heart. Excessive alcohol drinking weakens and enlarges the heart. Persons with heart murmurs caused by faulty valves or congenital heart disease are susceptible to endocarditis, a bacterial infection of the inner lining of the heart. Also, certain viruses can cause myocarditis, inflammation of the heart muscle, and pericarditis, inflammation of the outer lining of the heart.

Blood Vessel Disorders

Thrombus, blood clot that remains attached to place of origin in blood vessel.

Atherosclerosis, the thickening and hardening of arterial walls, may occur in many arteries. Cholesterol and other fats that form in the process obstruct the affected arteries and, at times, produce a thrombus, or clot, in them. Sometimes, these clots break away, especially from the heart, and embolize, or travel to some other part of the circulatory system. There, they can block a blood vessel and keep oxygen away from a vital body part. Embolism in the brain, for example, can cause a stroke.

Aneurysm, bulging and thinning of some point in the wall of a blood vessel (usually an artery) or of the heart because of arteriosclerosis (“hardening of the arteries”), embolism, infection, or physical injury; element common to all true aneurysms is injury to the media; after the aneurysm has developed it tends to grow, with danger that the vessel wall will rupture; treatment of aneurysm involves surgical removal of the diseased section of artery and its replacement with a plastic graft.

Aneurysm occurs when the walls of a large artery, especially the aorta, become weak and balloon out. Atherosclerosis can cause an aneurysm. So can syphilis. The venereal disease can also make the aortic valve leak.

Varicose veins, bulging veins in the leg, develop when the walls of the veins weaken. The condition may be inherited or may stem from phlebitis, an inflammation of the veins. Phlebitis may trigger clots in the veins, which sometimes break away, travel to the lungs, and form a pulmonary embolus. Drugs used to prolong clotting time often correct clotting disorder.


Cancer the collective name for any of the dangerous tumours, or growths, that can arise in the body is the second ranking cause of death in the United States and Canada. It claims more than 460,000 lives each year in the United States; some 87,000 each year in Canada. There are more than 100 different kinds of cancer.

Cancer is characterized by rampant, abnormal cell growth. If this occurs within a vital organ or tissue, normal function will be impaired or halted, with possibly fatal results. Cancer called sarcoma can arise in muscle, bone, connective tissue, blood vessels, and fatty tissue.

Cancer called carcinoma can arise in skin cells and in cells that line the body’s cavities and organs. Abnormal proliferation of white blood cells is called leukaemia An aberrant tumour in the body’s lymphatic tissue is a lymphoma. Cancer can strike many parts of the body. Some cancerous tumours are fast growing; they may double their size within a month or so. Others are slow growing and may not spread for many months or even years.

The Cancer Process

Tumours, or neoplasms, are purposeless bulges of excessive cell growth in tissue. If they are local and harmless, they are benign. If they can spread to other tissues and cause the body harm, they are malignant. Malignant tumours are cancerous.

Cell multiplication goes on normally in the human body for replacement of dead cells, but in cancer the multiplication goes somehow awry. Local malignant tumours often can be removed by surgery, thus ending the problem. However, if the cancer cells are not destroyed by surgery or other means, they may metastasise, or leave the local site and spread to other parts of the body. When they do metastasise, the entire body can succumb to the disease.

Cancer is believed to begin with one wildly multiplying cell in a given tissue. The process so resembles the action of cells in an embryo as they divide and shape the body that scientists think that cancer is tied in with the basic chemistry of the cell. After the embryonic cells have performed their tasks, certain chemical repressors lock up portions of DNA in genes in the cell nucleus.

These repressed pieces of DNA no longer trigger the biochemical reactions associated with rapid embryonic cell division. Thus, the seeds of cancer might be in everyone’s body. Then, at some time in the future, an event such as virus infection, radiation intake, inhalation of a carcinogen, or cancer-causing chemical, or an imbalance of hormones might free the genes and permit a mature body cell to revert back to an embryonic like cell. One theory even holds that the gene-bearing chromosomes of normal cells have certain sections capable of making virus like particles. These particles could then infect neighbouring cells and make them produce more particles, until many cells were proliferating wildly.

Tumour angiogenesis factor (TAF), substance that causes rapid growth of tiny blood vessels.

Cancer cells produce antigens against which the body reacts with antibodies. Small pockets of cancer cells, called silent cancers, might constantly be springing up in a person’s body, only to be destroyed by the body’s immunity system before they could do any harm. If the antibodies are ineffective, however, the cell mass grows to the size of a pinhead. Unless it gets enough blood, the pinhead mass will not get bigger. However, such tumours can give off a substance called tumour angiogenesis factor, which “fertilizes” rapid growth of tiny blood vessels into the tumour Then, it starts growing again because it has an ample supply of food from the blood. When it grows large enough to interfere with a vital body activity, the sufferer dies.

Treatment of Cancer

Cutting out the cancerous tissue through surgery is probably the most effective way of fighting cancer, as long as it has not had a chance to spread. Radiation treatment using radioactive cobalt or radium salts is another method of inhibiting the spread of cancer. Certain anticancer drugs hinder the growth of cancer cells and prevent their spread.


There are seven warning signs of cancer. If you notice any of these symptoms be sure to call them to the attention of a physician:

1. A sore that does not heal.

2. A lump or thickening anywhere in the body.

3. Nagging hoarseness or cough.

4. Unusual bleeding or discharge.

5. Persistent indigestion or difficulty in swallowing.

6. A change in bowel or bladder habits.

7. A change in a wart or a mole.

Early detection of cancer through annual physical examinations has been effective in reducing cancer death rates. In its early stages, cancer can often be stopped before it spreads.

Surgery performed early enough can remedy most women suffering from cancer of the uterus or cancer of the ovaries. A simple test, called the Pap smear, given by a physician at regular intervals can indicate the presence of precancerous uterine tissue before it becomes dangerous.

Lung cancer, which is often linked with cigarette smoking, affects males more than any other type of cancer; and it is spreading rapidly among female smokers as well. The next most frequent type among males is prostate cancer. Cancer of the breast is the leading type of cancer among females. Cancer of the colon/rectum ranks second. A woman can frequently discover breast cancer herself before it becomes serious by regular examination of her breasts for lumps.

Posted 2012/03/24 by Stelios in Education

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ANIMALS (Part 1 of 4)   Leave a comment


DEFINITION: 1 any living organism, excluding plants and bacteria: most animals can move about independently and have specialized sense organs that enable them to react quickly to stimuli: animals do not have cell walls, nor do they make food by photosynthesis 2 any such organism other than a human being, esp. a mammal or, often, any four-footed creature 3 a brutish, debased, or inhuman person 4 [Colloq.] a person, thing, concept, etc. thought of as a kind or type [today’s athlete is another animal altogether]

All living things are divided into two main kingdoms the animal and plant kingdoms and two or three other kingdoms that include bacteria, blue-green algae, and one-celled creatures with definite nuclei. What is the difference between a horse, for example, and grass? A horse moves about in the pasture eating grass. It trots toward you when you offer it a lump of sugar and shows pleasure when you stroke its head. The grass, however, is rooted to one place. It does not respond behaviourally to people or to the horse in any way.

Animals Move About and Sense Surroundings

Adult animals move freely from place to place during at least one phase of their life. Plants usually cannot move unless a force, such as the wind, causes them to move.

Most animals move freely from place to place and can sense their surroundings; that is, they can taste, smell, hear, see, and touch. Certain simple animals, such as the corals and barnacles, spend most of their lives fastened to one spot, but they are able to swim freely when they are young. Even these rooted animals have parts that move in order to capture food. Plants, however, cannot shift about at their own will. They react to heat, light, chemicals, and touch, but their responses are involuntary and automatic, quite different from those of animals.

All living things are made of cells. The walls of plant and animal cells are different.

Cellulose, complex carbohydrate consisting of 3,000 or more glucose units; basic structural component of plant cell walls; 90% of cotton and 50% of wood is cellulose; most abundant of all naturally occurring organic compounds; indigestible by humans; can be digested by herbivores, such as cows and horses, because they retain it long enough for digestion by micro organisms present in their digestive systems; also digestible by termites; processed to produce papers and fibres; chemically modified to yield plastics, photographic film, and rayon; other derivatives used as adhesives, explosives, thickening agents, and in moisture-proof coatings.

All living things are made up of cells of protoplasm. They may consist of a single cell, as does an amoeba, or billions of cells, as do trees and horses. The cell wall of a plant is composed of a woody material called cellulose. No true animal contains cellulose. Animal cells are bounded by a membrane composed chiefly of fat and protein.

Green plants make their own food. With the aid of the green substance called chlorophyll, they use the energy in sunlight to change carbon dioxide and water into carbohydrates and other food materials. No true animal contains chlorophyll.

Animals must eat, either directly or indirectly, the food manufactured by members of the plant kingdom. A horse cannot stand in the sun and wait for its body to make fat and proteins. It must move about the pasture in search of green grass. Even meat eaters for example, lions live on animals, such as zebras, which in turn subsist on plants.

The Variety of Animal Life

More than a million different kinds of animals inhabit the Earth. The exact number is not known, for new kinds are continually being discovered. They live in the seas, from the surface down to the black depths where no ray of light penetrates. On mountaintops and in deserts, in mud and in hot pools some form of animal life may be found.

Animals are infinitely varied in form, size, and habits. The smallest animals are bits of protoplasm that can be seen only with a microscope. The largest, the blue whales, may be more than 100 feet (30 meters) long and weigh 300,000 pounds (136,000 kilograms).

Some of the most familiar animals, such as dogs, birds, frogs, and fish, have a backbone and a central nervous system. They are called vertebrates, meaning animals with backbones. Animals without backbones are called invertebrates and include arthropods, worms, molluscs, and many other groups. Most of the vertebrates and many invertebrates have a head where sense organs are concentrated and have legs, wings, or fins for locomotion. Vertebrates and many invertebrates, such as the arthropods and worms, have bilateral, or two-sided, symmetry. This means that they have two mirror-image sides (a right side and a left side), distinct upper and lower surfaces of the body, and a distinct front and rear.

Some invertebrates, such as jellyfish, sea anemones, and starfish, display radial symmetry, in which the parts of the body are arranged around a central axis, similar to a wheel. Animals with radial symmetry live in marine or freshwater aquatic environments. Some drift with the currents, unable to swim in any definite direction. Others become attached to a solid object by one end and float with the mouth end upright. Tentacles arranged in a circle around the mouth sweep in food particles and ward off enemies.

One-celled animals called protozoans live in fresh and salt water. Many are shapeless creatures and cannot swim toward their food. They move along by squeezing out a finger like projection from the body. This is called a pseudopod, from the Greek meaning “false foot.” The pseudopod fastens to something solid, and the rest of the body flows into the fastened projection. The amoeba also moves in this manner. One-celled animals are very small. They are single blobs of liquid enclosed in a thin membrane and as such cannot attain a large size or a very definite shape.

Animals with Outside Skeletons and Feet

Molluscs have soft bodies that are not divided into specialized sections such as head, thorax, and abdomen. Many molluscs are enclosed in hard, hinged shells. Snails have a single large, fleshy foot located on the stomach side.

The heads of the octopus and the squid are surrounded by a circle of eight or ten tentacles that act as arms and feet. Oysters, clams, mussels, and scallops all have a single axe-shaped foot which they burrow into sand.

Most molluscs do not move around efficiently. Oysters fasten themselves to something solid and settle down for life, letting food drift to them. Scallops may move in zigzag leaps by clapping their shells together.

Joint-Legged Animals

Arthropod, animal of the phylum Arthropoda comprising invertebrates with external skeleton, segmented body, and jointed appendages.

Joint-legged animals, or arthropods, have bodies divided into segments that have specialized functions. These animals also have many jointed legs. Most arthropods are covered with a jointed skeleton made of a horny material. This outside skeleton is lighter than the shells of the molluscs The legs and muscles and many organs of the arthropod are attached to the outside skeleton.

The arthropods include insects, lobsters, crabs, centipedes, millipedes, scorpions, and spiders. They can run, jump, swim, and crawl. Some live mostly on land, while others live mostly in water. Many of the insects have wings and can fly. Arthropods inhabit most of the Earth’s environments, from the poles to the tropics, and are found in fresh and marine water and in terrestrial habitats.

How Back boned Animals Move

Vertebrates move through water and air and over the ground with great speed and skill. Birds, with their feathered wings, are the best fliers. Fish are the best swimmers. However, other vertebrates also can fly and swim. Bats fly on wings of membrane like skin. The flying squirrel glides on a broad membrane between its legs. The flying fish soars over the surface of the ocean by using its fins. Neither the fish nor the squirrel can soar great distances, however.

Some turtles swim with paddle like front legs. Some water birds can swim underwater with their wings. The mud skipper and walking catfish are fishes that walk on mud by pulling themselves along on their front fins.

Frogs, kangaroos, various cats, and some fishes are superior jumpers. Salmon leap up waterfalls when they travel from the sea to their home streams to lay their eggs. Tarpon, swordfish, and sailfish make great leaps out of the water when pursuing their prey or trying to escape an enemy.


Animals breathe in different ways:

  • Some animals, like amoeba and sponges, let oxygen move through their cell walls.

  • Fish and tadpoles breathe with their gills.

  • Insects bring air in through pores, or holes, called spiracles.

  • Mammals, birds, and reptiles breathe with lungs.

All animals must take in oxygen in order to change food into a form that the body can use. One-celled animals that live in water absorb oxygen directly through their membranes. The sponge is a very simple many-celled animal. The surface of a sponge is covered with millions of tiny pores. Water bearing dissolved oxygen and minute food particles flows through the pores and out of the opening at the top of the sponge.

Fish and tadpoles breathe by means of gills. Insects and caterpillars take air into the body through breathing pores called spiracles.

Mammals, birds, and reptiles obtain oxygen from the air. They take it into the lungs, and the oxygen passes through membranes in the lungs into particles called red blood cells. The bloodstream then carries the oxygen to all parts of the body. Amphibians have lungs, but they also have thin, moist skins that absorb oxygen directly.


Sea squirt, a tunicate or saclike marine animal, so called from its habit of ejecting water when touched; belongs to the phylum Chordata.

Hydra, primitive water animal of the class Hydrozoa and the genus Hydra.

All animals reproduce their own kind. One of the most primitive forms of reproduction is by fission, in which the individual organism divides to produce a replica of itself. Some animals, such as sea squirts, reproduce by budding: lumps appear along a branchlike organ and develop into young sea squirts. Sea squirts, sponges, corals, and other creatures that bud often remain together and form large colonies. The hydra also reproduces by budding, but in time the young bud separates and goes off to live alone.

Most animals reproduce by means of eggs from the female that are fertilized by sperm from the male. The eggs of some species are deposited in a nest or in some other manner before hatching. Most species of mammal and some species of reptile and fish bear their young alive, the fertilized eggs being retained within the body of the female.

The types of reproductive behaviour among animals are almost as varied as the kinds of animals themselves. Some species, such as most insects and turtles, deposit their eggs and give them no further attention. In colonies of the social insects, such as ants and bees, a single female lays all of the eggs, and workers provide care and nourishment for the developing young in the nest. The females of some reptiles, such as the king cobra and the blue-tailed skink, and amphibians, such as the marble salamander, stay with their clutch of eggs until they hatch but provide no protection or nourishment for the young. Some fish guard their young after they are born. Crocodilians protect the eggs before hatching and the young for several months afterwards. Many birds provide not only protection but also nourishment for the developing young. Mammals, which feed their young with milk produced by the mother, provide care for their young much longer than do other classes of animals.

Posted 2012/01/29 by Stelios in Education

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