<text><span class="style10">he Immune System (4 of 4)</span><span class="style7"></span><span class="style10">AIDS AND THE IMMUNE SYSTEM</span><span class="style7"> The </span><span class="style23">human immunodeficiency virus</span><span class="style7"> (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 </span><span class="style23">opportunistic infections</span><span class="style7">, 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. An AIDS virus (orange) budding from the plasma membrane (blue) of an infectedT-lymphocyte, viewed through a false-color electron microscope.</span></text>
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<text><span class="style10">n AIDS virus</span><span class="style7"> (orange) budding from the plasma membrane (blue) of an infected T-lymphocyte, viewed through a false-color electron microscope.</span></text>
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<text>ΓÇó REPRODUCTIONΓÇó RESPIRATION AND CIRCULATIONΓÇó GLANDS AND HORMONESΓÇó NON-INFECTIOUS DISEASESΓÇó INFECTIOUS DISEASESΓÇó PREVENTING DISEASE</text>
