By 1983, researchers linked the human immunodeficiency virus, HIV, to AIDS. It targets lymphocytes in lymph nodes, and CD4 helper T cells. In most cases, HIV is transmitted through blood or semen.

HIV is a retrovirus, which means that its genetic material is RNA. The virus attaches to CD4 receptors on helper T cells, and sends in its RNA. A viral enzyme, reverse transcriptase, catalyzes construction of a DNA strand complementary to the viral RNA. The initial viral DNA strand replicates to form a DNA double helix, which enters the T cell's nucleus.

Using the invaded cell's protein synthetic machinery, the virus replicates, filling the cell with HIV, RNA and proteins. Not only can the dying T cell no longer release cytokines or stimulate B cells to manufacture antibodies, but it bursts, unleashing many new HIV particles. HIV replicates at an astounding rate, producing 3 billion new particles a day, from the start of infection. Immune system cells can remove only 100 million to a billion per day, so soon the system becomes overwhelmed. Opportunistic infections set in. A danger sign in HIV infected people is a falling CD4 T cell count, signaling collapse of the coordination of the immune response.

Treatment

AIDS treatment includes treating individual opportunistic infections and cancers as they arise, and using palliative measures to make life more comfortable. A drug called azidothymidine (AZT) can delay symptom onset, but once symptoms begin they tend to be more severe. AZT does not extend survival, and its side effects may be intolerable. However, it does sharply decrease the rate of transmission from a pregnant woman to a fetus. AZT is normally given to people who are infected with HIV and whose CD4 helper T cell counts dip below 500/mm³. It inhibits the ability of HIV to replicate. Many other AIDS drugs are in development and testing stages, including existing drugs such as acyclovir, interferon, thalidomide, and amphotericin B, as well as many new compounds. Because HIV mutates into drug-resistant forms rapidly, from the very beginning of infection, a combination of drugs may be the best approach to treatment.

The key to conquering AIDS may lie in the long-term survivors and in a few children whose HIV infections have cleared. These children had HIV genetic material present during infancy, but do not have it several years later, and are healthy. By learning how their immune systems function, and how the virus is impaired, we may finally be able to defeat this foe.

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