The lymph nodes, present throughout the body, filter the lymph - a clear fluid that drains from the body tissues. The lymph collects in the vessels of the lymphatic system, and eventually returns to the blood. It first passes through the lymph nodes and any microorganisms or cancer cells are filtered out. If an infection is present, lymphocytes respond by multiplying, which accounts for the swelling of the nodes - for example, in the armpits and under the lower jaw - that sometimes occurs. Vast numbers of other types of white blood cell, e.g. microphages (neutrophils) and macrophages, can engulf and destroy microorganisms. They also destroy red blood cells that have reached the end of their 120-day life span. These so-called phagocytic cells are found in the tissues, lymph nodes and spleen.

Antigens and Antibodies

An antigen is any foreign substance - for instance, the protein on the coat of a bacterium - that can stimulate an immune response. When T cells meet antigens they respond by multiplying and dividing, releasing molecules that stimulate other cells of the immune system (including other T cells) to grow.

There are may different kinds of T cells. Cytotoxic T cells can recognize and kill cells infected with viruses. T-helper cells can help macrophages to kill microorganisms. T-helper cells also have an important role in stimulating B cells.

Once stimulated, a B cell multiplies. Its offspring mature into plasma cells, which secrete antibodies. These are specialized molecules that can latch on to antigens and help the rest of the immune system eliminate the foreign particle. There is potentially an infinite variety of anti-bodies, one for every conceivable antigen. Once a B cell is stimulated, the result is a clone of plasma cells, all dedicated to manufacturing the antibody that recognizes the antigen in question.

Immunity and Memory

A few of the cells that result when a B cell divides in response to an antigen are so-called memory cells. These remain in the body for life: when the individual meets the same antigen again, they are ready to respond, faster and with more force than before. This explains why people who have one attack of rubella (German measles), for example, are immune to subsequent infections by this virus.

Immunization works on this principle. Vaccines aim to prime the immune system to recognize disease-causing organisms, so that it will spring into action when it encounters the microorganisms concerned. Several vaccines consist of bacteria or viruses that have been killed or weakened; they provoke a protective immune response, but no longer have the capacity to cause the disease.

The Modern View of AIDS

The human immunodeficiency virus (HIV), which causes AIDS (Acquired Immune Deficiency Syndrome), strikes at the heart of the body's defenses. The virus destroys the immune system, including the very cells that should be capable of eliminating it. In infected people the virus is found in the blood, semen and - to a lesser extent - vaginal secretions. It can be transmitted if any of these fluids gains access to another person's blood stream. This can occur through sexual contact involving exposure to semen, as the virus could enter even the most minute cut or abrasion in the vagina or the more delicate rectum. It can also be spread among drug users via shared hypodermic needles.

People infected with HIV may remain apparently well for many years. After a variable incubation period, which may average as long as 9 or 10 years, many affected people - although no one knows exactly what proportion - will go on to develop AIDS. The disease develops when the individual's level of T-helper cells falls drastically.

Without the T-helper cells, which orchestrate many of the components of the immune system, it becomes impossible for the body to fight off infectious agents. The person falls prey to a variety of opportunistic infections, so called because they have taken advantage of the failing immune system. Certain cancers, including the skin cancer known as Kaposi's sarcoma, may also develop in AIDS.

Medical scientists studying AIDS have been puzzled by the observation that HIV seems to infect only a very small proportion of T-helper cells circulating in the blood. Even if these infected cells died, the body would produce T cells at such a rate that they would easily be replaced.

One theory to explain the loss is that some effects of the virus on uninfected cells may be to blame. For example, viral proteins circulating in the blood may attach themselves to the cells that HIV attacks. Cytotoxic T cells may then see these cells as infected (even though they are not) and kill them.

HIV infects not only T-helper cells, but also macrophages. Sometimes the virus can multiply within the macrophages to the point where the cells are bursting with viruses. Possibly, the primary defect in AIDS may lie with the macrophages.