Human immunodeficiency virus (HIV) can infect your blood and cause acquired immunodeficiency syndrome (AIDS). HIV infects and destroys a particular type of white blood cell, a T lymphocyte, which has a special receptor (attaching) site called CD4 on the outside, so it is called a CD4 + T lymphocyte. This white blood cell is part of the immune system of the body. According to the National Institute of Allergy & Infectious Diseases, you should have about 800 to 1200 of these CD4+ T lymphocytes in each cubic millimetre (mm3) of blood. The test that measures this is called a "CD4 count" or a "T-cell count." Ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) are found in living cells. DNA (double-stranded) carries genes and the blueprint for cell function, but RNA (single-stranded) has multiple functions, including carrying genetic instructions and causing chemical reactions. HIV contains RNA while a T lymphocyte contains DNA.
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The Aaron Diamond AIDS Research Center outlines the complex process by which HIV infection and replication (multiplication) occurs. HIV enters your blood and finds a CD4+ T lymphocyte, attaches to the special receptor site on the outside of the cell and releases its RNA inside the lymphocyte. This HIV RNA then converts into HIV DNA and enters into the lymphocyte's own DNA where the HIV DNA can remain hidden for weeks, months, or years as a provirus until it activates. This provirus carries the blueprint for HIV. You now have an HIV infection but the viral load (the count of virus) in your blood is low because the virus is not replicating, and your CD4 count is still normal because the HIV is waiting in the cell and not causing damage.
The HIV DNA hidden in the lymphocyte activates and the RNA it carries begins to form viral particles, which are released into the cell. Part of the wall of the cell wraps around the viral particles, creating a casing for each one, and these then bud out from the cell and eventually breaks loose to form new viruses, destroying the T lymphocyte in the process. The new viruses go in search of more CD4+ T lymphocytes to invade and then repeat the process. At this point, your viral load starts to increase because there is more HIV in your blood, and your CD4 count starts to fall because your T lymphocytes are being destroyed. As your CD4 count falls, you are at risk for opportunistic infections, such as pneumonia, tuberculosis, and herpes because you don't have enough CD4+ T lymphocytes to protect your body. You are diagnosed with AIDS when your CD4 count falls below 200.
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