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.

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.

HOW THE BODY FIGHTS DISEASE

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.

Other Growth Changes

Some alterations in tissue growth are not cancerous. Atrophy, for example, is a lessening in size. It is the shrinking of cells or tissues for various reasons. Starvation, for instance, causes atrophy of the adipose, or fatty, tissues. Disuse of a body part may also lead to atrophy. When a fractured arm is placed in a cast, the arm’s muscles decrease in size from lack of use.

Compensatory hypertrophy, in medicine, condition that results when one of a paired set of organs, such as a kidney or a lung, is removed and the remaining organ increases in size.

Hypertrophy is an increase in size of individual cells or fibres It results in an enlargement of the body part containing these muscles or fibres Hypertrophy of the heart has already been discussed. Compensatory hypertrophy is best seen in paired organs. When a diseased kidney is removed from the body, the remaining kidney grows larger because it now must do the work of two kidneys.

RESPIRATORY SYSTEM DISEASES

The lungs are spongy organs through which vital oxygen enters the body and needless carbon dioxide exits. Oxygen and carbon dioxide are exchanged in and out of capillaries in the many tiny air sac’s, or alveoli, in the lungs. Although the breathing passages have defences against invading germs and irritants, the lungs can be stricken by a number of serious diseases.

Chronic bronchitis is a disease that results from infection of the air passages by bacteria or viruses. It is marked by cough and increased production of sputum, an accumulation of saliva, mucus, and pus. Air pollution and cigarette smoking both can aggravate the malady.

Tuberculosis, bacterial disease most frequently affecting lungs; associated with fever and loss of weight; commonly transmitted through the air (droplet infection) but also from drinking unpasteurized milk obtained from infected cows.

Tuberculosis is a complicated disease that most often strikes the lungs. The bacilli that cause it grow from place to place in the lung, leaving cavities in the unoccupied sites.

Symptoms of tuberculosis may include weight loss, fever, chest pain, cough, and sputum. After the active infection is arrested, a period follows when the disease may break out again. Tuberculosis is treated with isoniazid and other drugs.

Pneumonia, or acute infection of the lungs, may occur suddenly in a seemingly healthy person. It is usually marked by fever, cough, and chest pain. Lung X rays show patches of inflammation. Though once quite fatal, the threat of pneumonia has been reduced as a result of antibiotic treatment.

Pleurisy, an inflammation of the pleura; caused by infection, injury, or other chest diseases.

Pleura, the serous membrane that covers the lungs, lines the walls of the thorax, and is reflected upon the diaphragm.

Pleurisy is severe chest pain accompanying each deep breath in a person with an inflamed pleura, the twin membrane around each lung and lining the chest cavity. Pleurisy can attend pneumonia or result from direct infection of the pleura.

Emphysema is a serious lung disease that follows destruction of the elastic and connective tissue fibres supporting the lung. It is linked with advancing age. Certain forms of emphysema are inherited. Heavy cigarette smoking and long exposure to air pollutants seem to encourage the disease. A person with emphysema, lacking sufficient lung elasticity, wheezes and has trouble breathing. Furthermore, air movement in the lungs is reduced and the patient is easily fatigued because he fails to get enough oxygen or get rid of enough carbon dioxide.

Asthma is the wheezing or whistling sound that accompanies each breath when the air passages contain too much mucus. It may follow lung infection or result from an allergic reaction that causes muscle spasms and swelling in the air passages.

Acute pulmonary oedema results when fluid quickly accumulates in the lungs and fills the alveoli. The fluid build-up is caused by heart trouble that, in turn, produces back pressure in the pulmonary veins and the left atrium of the heart to which they carry oxygen-rich blood from the lungs. A person suffering acute pulmonary oedema is suddenly breathless and turns blue because of oxygen-poor blood. The condition is treated with oxygen, digitalis to strengthen heart action, and diuretics to speed fluid removal by the kidneys.

Pneumothorax, presence of air in the usually air-free pleural space between the lungs and chest walls.

Pneumothorax occurs when air gets into the chest between the pleural lining. The lung then collapses. A collapsed lung may occur when the chest is pierced in some way or when an abnormal bleb, or blister, on the lung surface bursts.

Lung abscess is an accumulation of a mass of pus inside the lung. A lung abscess can increase the seriousness of pneumonia and other lung infections, especially in chronically ill persons.

Hyaline membrane disease is a disorder of some prematurely born infants. The alveoli of afflicted babies are lined with a protein material, limiting the amount of oxygen their blood can receive. The disease is often fatal.

Histoplasmosis is a fungus infection of the lungs. Fungi lodge in the lungs and multiply until body defences wall them off. In some areas it was once called “summer flu” because its symptoms resemble those of influenza. Serious cases involve weight loss and a long convalescent period.

Silicosis, disease of the lungs, caused by inhaling tiny sharp particles of stone dust; fibrous tissue forming around particles causes cough, shortness of breath, and weakness.

Pneumoconiosis means “dust disease.” It can strike miners and industrial workers who inhale damaging amounts of dust. One of the most serious is silicosis, which results from inhaling quartz dust. Another, anthracosilicosis, arises from inhalation of coal and quartz dust.

SKIN DISEASES

Because of its location the skin is perhaps more susceptible to disease than any other body organ. Even so, it is marvellously designed for its particular jobs of protecting the inner body against harm from the outside surroundings, receiving clues about what is happening externally, and keeping the body cool by means of the evaporation of sweat produced by its sweat glands. The skin is thick, leathery, and tough enough to prevent mechanical injury to the body. It is also covered with a barrier of dead cells that block harmful chemicals from getting into the body.

The skin is richly supplied with nerves that enable the perception of pain, touch, heat, and cold. Blood vessels in the skin can either contract or expand in response to nerve signals. A person’s emotional state can often be observed through changes in skin colour Shame or rage reddens the skin; fear blanches it. The skin may react to disease in a great many ways including formation of blisters, pimples, ulcers, tumours, and by haemorrhage

Blackheads are an accumulation of a horny material in special follicles of the face. The characteristic black dots in blackheads are not dirt but melanin, the pigment responsible for skin colour

Acne is an outcropping of blackheads or pimples on the face of an adolescent. It is brought on by hormonal changes that accompany sexual maturity. It is not caused by food, emotions, or uncleanliness. Antibiotics are available for the treatment of severe acne, but most cases respond well to local application of a peeling agent.

Warts are horny growths caused by virus infection. They are spread from person to person. Although warts cannot be prevented, they can be burned away with an electric needle or a caustic chemical such as nitric acid.

Hives, or urticaria, are itchy, whitish elevations of the skin. They appear and disappear rapidly. Hives are often the result of an allergic reaction to certain foods or medicines. Persons who suffer severe cases of hives can receive a series of desensitizing shots. Antihistamine drugs sometimes can relieve a bout of hives.

Birthmark, a skin blemish, result of an overgrowth of blood vessels.

Birthmarks are the result of an overgrowth of blood vessels. They usually show up after birth as port-wine-coloured stains or strawberry-coloured marks. The strawberry marks may eventually disappear but at times can be destroyed quickly by the application of extreme cold. Port-wine stains and other long-lasting skin blemishes can be concealed by special cosmetics.

Eczema, or dermatitis, is a superficial inflammation of the skin. It can be an allergic reaction to poison ivy, dyes, or drugs. It can be provoked by such irritants as acids, solvents, or excessive use of soap or detergents. Sunburn can also cause eczema. Some forms of it, such as infantile eczema and seborrhoeic dermatitis, stem from an unknown cause. Nonetheless, nearly all types of eczema can be relieved by the application of corticosteroid creams.

Athlete’s foot is a fungus infection of the skin between the toes. The infected area is scaly, moist, and itchy. It usually has a disagreeable smell. Athlete’s foot can be relieved when anti fungal drugs are applied to the infected skin each day. Fungus infections that cause a loss of hair or nails must be treated with griseofulvin, an antibiotic.

Bacterial infections such as the psoriasis caused by “staphylococci” germs are rare because of modern standards of hygiene and sanitation. However, the bacterial disease gonorrhoea, which passes between the skin of the sex organs, has risen to epidemic proportions among teenagers in recent years. This and other bacterial infections of the skin are remedied with antibiotics.

NERVOUS SYSTEM DISEASES

The nervous system is the quick communication system of the body. Information from the outside world enters the body through the sense organs and is sent to the spinal cord for instant response or is relayed to the brain for further processing.

Nerves and the membranes that protect portions of the nervous system are susceptible to breakdown or infection. Sometimes, the organisms that cause such diseases as mumps or infectious hepatitis can infect the nervous system, too.

Nervous System Infections

Meningitis is an inflammation of the meninges, or membranes around the brain and spinal cord. It can occur through viruses, bacteria, fungi, or yeasts that get into the nervous system. Meningitis is a serious disease and can be fatal.

Shingles, inflammation of certain nerve tissue caused by virus herpes zoster.

Shingles, or herpes zoster, is a virus-caused inflammation of certain nerve tissue. Painful skin bumps occur over the line of the inflamed nerve or its branches. Shingles and chicken pox are both caused by the same virus.