Lack of one or more components of the immune system results in immunodeficiency disorders. These can be inherited, acquired through infection or other illness, or produced as an inadvertent side effect of certain drug treatments.

People with advanced cancer may experience immune deficiencies as a result of the disease process or from extensive anticancer therapy. Transient immune deficiencies can develop in the wake of common viral infections, including influenza, infectious mononucleosis, and measles. Immune responsiveness can also be depressed by blood transfusions, surgery malnutrition, and stress.

Some children are born with defects in their immune systems. Those with flaws in the B cell components are unable to produce antibodies (immunoglobulins). These conditions, known as agammaglobulinemias or hypogammaglobulinemias, leave the children vulnerable to infectious organisms; such disorders can be combated with injections of immunoglobulins.

Other children, whose thymus is either missing or small and abnormal, lack T cells. The resultant disorders have been treated with thymic transplants.

Very rarely, infants are born lacking all the major immune defenses; this is known as severe combined immunodeficiency disease (SCID). Some children with SCID have lived for years in germ-free rooms and "bubbles." A few SCID patients have been successfully treated with transplants of bone marrow (Bone Marrow Transplants).

The devastating immunodeficiency disorder known as the acquired immunodeficiency syndrome (AIDS) was first recognized in 1981. Caused by a virus (the human immunodeficiency virus, or HIV) that destroys T4 cells and that is harbored in macrophages as well as T4 cells, AIDS is characterized by a variety of unusual infections and otherwise rare cancers. The AIDS virus also damages tissue of the brain and spinal cord, producing progressive dementia.

AIDS infections are known as "opportunistic" because they are produced by commonplace organisms that do not trouble people whose immune systems are healthy, but which take advantage of the "opportunity" provided by an immune defense in disarray. The most common infection is an unusual and life-threatening form of pneumonia caused by a one-celled organism (a Protozoa) called Pneumocystis carinii. AIDS patients are also susceptible to unusual lymphomas and Kaposi's sarcoma, a rare cancer that results from the abnormal proliferation of endothelial cells in the blood vessels.

Some persons infected with the AIDS virus develop a condition known as AIDS-related complex, or ARC, characterized by fatigue, fever, weight loss, diarrhea, and swollen lymph glands. Yet other persons who are infected with the AIDS virus apparently remain well; however, even though they develop no symptoms, they can transmit the virus to others.

AIDS is a contagious disease, spread by intimate sexual contact, by direct inoculation of the virus into the bloodstream, or from mother to child during pregnancy. Most of the AIDS cases in the United States have been found among homosexual and bisexual men with multiple sex partners, and among intravenous drug abusers. Others have involved men who received untreated blood products for hemophilia; persons who received transfusions of inadvertently contaminated blood-primarily before the AIDS virus was discovered and virtually eliminated from the nation's blood supply with a screening test; the heterosexual partners of persons with AIDS; and children born to infected mothers.

There is presently no cure for AIDS, although the antiviral agent zidovuzine (AZT) appears to hold the virus in check, at least for a time. Many other antiretroviral drugs are being tested, as are agents to bolster the immune system and agents to prevent or treat opportunistic infections. Research on vaccines to prevent the spread of AIDS is also under way.