The thymus gland, two oval lobes just behind the breastbone, is large in early infancy, as pictured, but by age eight or ten, begins to atrophy. In adults, the thymus has shrunk to about thumb-size. It is responsible for the development of he immune system. In infancy, it produces cells called lymphocytes that are coded to recognize and protect the body’s own tissues, while they trigger an immune response against invaders. Later, the lymph nodes, bone marrow, and spleen take over the job of producing lymphocytes.

How Does the Body Defend Itself Against Disease?


The body has very impressive armaments to ward off disease, ranging from the skin, which acts as a barrier to infection, to acids in the stomach that kill bacteria, to mucus in the respiratory and unrogenital tracts that carry off alien particles. But the most intricate – and until modern times – most mysterious defenses are those to be found in your blood and lymph nodes. Scavenger white blood cells and certain substances called antibodies in the blood are the warriors of your immune system, capable of destroying harmful invaders. The mechanisms of immunity are remarkably complex, and some scientists believe that an understanding and manipulation of antibodies and other substances in the blood will eventually revolutionize medical care.

Almost everyone is born with an intact but undeveloped immune system, which matures shortly after birth. During this maturation period, the infant’s immunity is supplemented by factors acquired from the mother’s blood an milk. Human beings are also naturally immune to many diseases that afflict animals. How efficiently your immune system works depends to some extent on heredity, but eating and working habits also have an influence on it.

What if the Body Cannot Fight Off the Agents of Disease?

There are many diseases for which the body does not develop immunity. Each person becomes resistant to a disease through acquired immunity is to get the disease. Every microorganism carries with it a specific antigen, which the body perceives as foreign. In response, the blood and the lymphatic system begin to manufacture antibodies, proteins that can neutralize the effects of a specific antigen.

If the body’s resistance is strong enough, the antibodies gradually overwhelm the invaders, and the disease subsides. From that time on, in the case of certain infections, the normal body is immune to attack by that particular disease.

How Do Vaccines Work?

Another way to gain active immunity is through artificial immunization by vaccination. This involves giving injections of vaccines that are developed from the cultures of disease-causing organisms that have been rendered harmless, or nearly so, in the laboratory. Whatever form the vaccine takes, it stimulates the body to produce its own antibodies and thus to acquire immunity. In some instances, the organism is killed by heat, formaldehyde, or ultraviolet light before the vaccine is prepared; the vaccines against whooping cough and typhoid fever are prepared this way. Many vaccines, however, must be made from live organisms if they are to give effective immunity. To make them safe, the organisms are first weakened, or by some other means. An example of this type is the Sabin oral polio vaccine. A third group of vaccines, which includes the ones against smallpox and tuberculosis, related but milder diseases.

When are Antitoxins and Gamma Globulin Used?

Some bacterial infections, chiefly tetanus, botulism and gas gangrene, cause disease by dumping poisons into the bloodstream. Both antitoxins and gamma globulin may be used to combat their effects.

Antitoxins are developed by injecting small amounts of bacteria into laboratory animals. Antibodies are taken from animal blood and purified before being injected into human beings. They confer immediate but short-lived immunity, lasting perhaps no more than six weeks. Booster shots of the same antitoxin may be given if the danger of disease recurs.

Unlike antitoxins, gamma globulin is derived from the blood of a human donor who has previously contracted a specific disease and developed antibodies against it. Gamma globulin is sometimes given to individuals exposed to a serious disease against which they have not been vaccinated. It is administered most commonly following exposure to hepatitis A or B virus, but is often effective in preventing mumps and measles as well.

What Happens When the Immune System Breaks Down?

Failure of the immune system can be very serious. One form of congenital immune deficiency results when the unborn infant’s bone marrow cannot produce the specialized white blood cells essential to the immune system. Immunologically helpless against all infections, the child usually dies shortly after birth from infections that it is unable to fight.

Acquired immune deficiency syndrome, or A.I.D.S., develops in healthy people after infection by a virus that destroys specific white blood cells. They succumb to various diseases, most notably cancer of the immune system and infections.

Autoimmunity is the condition in which the body’s immune system mistakes its own cells for foreign antigens. It then produces antibodies that attack and destroy healthy body tissue. There is strong evidence that rheumatoid arthritis, multiple sclerosis, and a form of hyperthyroidism may all be traceable to an abnormality of the immune system.

Triggering Immune Responses by Vaccination

Vaccination has a long history, beginning ages ago with the Chinese, who inhaled a powder made from smallpox victims’ tissue, and the Greeks and Turks, who scraped healthy people with living smallpox germs. The latter was tried in England in the early 1700s, but it was not until 1796 that English physician Edward Jenner found a safe use of the concept.

He noted that people who contracted mild cowpox were immune to smallpox, so he infected a patient with cowpox, hoping to protect him from smallpox. He succeeded, thus developing the first safe vaccination method in the Western world.

The use of cowpox vaccine in the 1800s worked but excited controversy and ridicule.

Edward Jenner showed his faith in vaccination by injecting his own son with cowpox, therefore immunizing the child against smallpox.

This injector gun, which can safely inoculate as many as 1,000 people an hour, is the latest word in vaccination.