TRANSMISSION OF HIV
Friday, April 22nd, 2011HIV is transmitted—that is, the virus enters the body—almost invariably by sexual contact, by blood-to-blood contact, or through pregnancy.
When HIV enters the body, it attaches itself only to certain sites on the walls of certain cells. The site on the cell wall is called the CD4 receptor, and the cell most commonly infected is called the CD4 cell. The CD4 cell is a white blood cell, or a lymphocyte. It belongs to a class of lymphocytes called T cells, which, along with B cells, are central parts of the immune system. (The CD4 cell is also called a T4 cell and a T-helper cell.) The CD4 cell’s job is to help coordinate the immune system’s defense against a variety of infectious diseases. HIV is carried by CD4 cells and other white blood cells to all parts of the body, including the brain. HIV also attaches itself to certain cells in the brain.
Once HIV attaches to a CD4 cell, it enters the cell. At this point, in a complicated series of events, the virus becomes part of the cell’s genes. Genes are composed of DNA, a molecule which is responsible for directing the reproduction of the cell. HIV is a virus and has only RNA, a molecule which is actually the mirror image of DNA but which cannot produce new viruses. HIV, however, is a retrovirus, meaning that it has a protein called reverse transcriptase. Reverse transcriptase allows the viral RNA to turn into a mirror image of itself; that is, it allows viral RNA to turn into viral DNA. This DNA then directs the infected cell to produce, not new CD4 cells, but new HIVs instead. The virus eventually destroys the CD4 cell, and the new viruses that have been produced then infect other CD4 cells. As CD4 cells are infected and destroyed, the immune system functions less and less effectively.
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