The human immunodeficiency virus (HIV)

Like other retroviruses, HIV infects the body, has a long incubation period (clinical latency), and ultimately causes the signs and symptoms of AIDS. HIV causes severe damage to the immune system and eventually destroys it. Initial reports of AIDS date back to 1981. However, current data suggest that AIDS has existed for at least several decades. Both HIV types 1 and 2 are capable of causing fatal AIDS, but infection with the latter generally results in a longer incubation period and a more indolent course of disease. HIV-2, initially endemic to west Africa, is spreading worldwide. HIV-2 demonstrates a closer genetic relationship and geographic distribution to the simian immunodeficiency virus (SIV), long endemic to central Africa, than to HIV-1. Sequencing studies of early HIV-2 isolates showed a 75% nucleic acid homology with SIV but only a 40 to 50% homology with HIV-1. Thus, it was hypothesized that HIV-2 may be the prototype virus that was originally transmitted from monkeys to man. Transmission of HIV can occur through contact with infected body fluids. It is currently assumed that the relatively cellular body fluids such as blood, semen, vaginal secretions, and breast milk are more effective in transmitting the virus than fluids deficient in cells such as saliva, urine, and tears. Transmission may occur across mucous membranes or broken skin during sexual intercourse but it also may occur via intravenous exposure such as through sharing infected needles with intravenous drug use, occupational exposure in the health care environment, or treatment with infected blood products. The immune system protects the body by recognizing antigens on invading bacteria and viruses and reacting to them. An antigen is any substance that induces a state of sensitivity and immune responsiveness. The immune system consists of lymphoid organs and tissues, including the bone marrow, thymus gland, lymph nodes, spleen, tonsils, adenoids, appendix, blood, and lymphatic vessels. All of the components of the immune system are vital in the production and development of lymphocytes or white blood cells. B- and T-lymphocytes are produced from stem cells in the bone marrow. B-cells stay in the bone marrow to complete the maturation process, but T-lymphocytes travel to the thymus gland to complete their maturation. It is in the thymus that T-lymphocytes become immunocompetent, multiply, and become more differentiated. The main function of B-lymphocytes or B-cells is humoral (antibody) immunity. Antibodies are effective against extracellular parasites such as bacteria. But T-lymphocytes kill HIV-infected cells. Cytotoxic CD8+ cells kill HIV-infected cells and cancer cells. The other type of CD8+ cells, T-suppressor cells, inhibits or suppresses immune responses. It is known that HIV activates T-suppressor cells. Normal CD8+ cell count is between 300 and 1000 cells in adults and children. Normal CD4+:CD8+ ratio is between 1.0 and 2.0. HIV consists of a cylindrical center surrounded by a sphere-shaped lipid bilayer envelope. There are two major proteins in this lipid bilayer, gp120 and gp41. The major function of these proteins is to mediate recognition of CD4+ cells, thereby enabling the HIV virus to attach to and invade the CD4+ cells. The inner sphere contains two single stranded copies of thegenomic material – ribonucleic acid (RNA) – as well as multiple proteins and enzymes necessary in the process of HIV replication and maturation: p24, p17, reverse transcriptase, integrase, and protease. l Kazhila Chinsembu is a lecturer in the Department of Biology, University of Namibia. Email: kchinsembu@unam.na

June 2006
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