A weakened immune system can be congenital (present from birth), or it may result from an inherited disorder or an infection. It also may occur as a side effect of drug treatment. People with weakened immune systems have difficulty fighting infections that are easily handled by a healthy immune system. When a person's immune system is ineffective, harmful microorganisms can thrive and multiply rapidly, causing life-threatening infections.
A small number of infants are born with defects in their immune systems. Infants born with defective B lymphocytes are unable to produce antibodies and therefore are vulnerable to a wide variety of infections. Doctors treat this type of disorder with injections of live antibodies. Some infants lack T lymphocytes because they were born without a thymus gland or because the thymus gland is small and abnormal. This disorder requires a thymus gland transplant. In very rare cases, infants are born without an immune system, a condition known as severe combined immunodeficiency disease. Children with this disease gained notoriety by living for years in germ-free rooms or "bubbles." A few of these children have been successfully treated with bone marrow transplants.
The best-known immunodeficiency disorder is acquired immunodeficiency syndrome (AIDS), which is caused by the human immunodeficiency virus (HIV). AIDS often leads to various otherwise rare diseases, such as Pneumo-
cystis carinii pneumonia and Kaposi's sarcoma (a form of cancer). These dis- 379
eases are called opportunistic diseases because they rarely occur in people with Immune healthy immune systems, but occur more in people with weakened immune sys- system tems, in whom conditions for development are favorable. HIV infection also damages the central nervous system (the brain and the spinal cord), which leads to dementia (progressive deterioration of mental functioning).
HIV infection may cause no symptoms in the early stages or can produce flulike symptoms, including fever, chills, fatigue, sweating at night, and a dry cough. Other symptoms include sudden unexplained weight loss and chronic diarrhea. Although some infected people may have no symptoms, they still can transmit the virus to others.
HIV is transmitted by sexual contact or by direct contact with infected blood or body fluids. HIV can be transmitted from an infected woman to a fetus during pregnancy or to an infant during childbirth or through breast milk. HIV also can be transmitted by sharing needles during intravenous drug use. In the past, some people were infected with HIV through blood transfusions or contaminated blood products. Today, however, all donated blood is routinely screened for HIV, and the blood supply in the United States is safe.
At present there is no cure for AIDS. Early diagnosis and treatment are vital. Certain medications that are currently available, when used in combination, seem to slow development of the disease. These medications include nucleoside analog reverse transcriptase inhibitors (such as zidovudine, also called AZT), which interrupt an early stage of virus replication, and protease inhibitors (such as indinavir, ritonavir, and saquinavir), which interrupt the same process at a later stage. Although these "cocktail" drug regimens typically cause unpleasant side effects such as nausea and diarrhea, when started early, they are enabling many people infected with HIV to live longer, more productive lives. Other drugs designed to combat the virus, bolster the immune system, and prevent or treat opportunistic infections that result from HIV infection are now being tested. Research on an AIDS vaccine is also under way. For more information on HIV and AIDS, see page 186.
Immunodeficiency can be an unwanted side effect of certain medications, such as those used to treat cancer. Anticancer drugs used during chemotherapy affect cells that divide rapidly, including the white blood cells (lymphocytes) that fight infection. Because of this, people undergoing chemotherapy may become more susceptible to opportunistic infections.
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