Primary impact of HIV on the immune system

In a healthy person, the normal range of certain white blood cells called T4 lymphocyte cells (helper cells) is usually between 600 and 1200 (T4) cells per cubic millimeter (cells/mm3). (The range depends on the test used.)

When human immunodeficiency virus (HIV) enters the bloodstream, it primarily infects T4 cells. Asymptomatic individuals infected with HIV usually have a lower than normal T4 cell count, and people with AIDS generally have between 0 and 500 T4 cells/mm3.

The number of T8 lymphocyte cells (suppressor cells) in an HIV-infected person usually stays about the same. Because the number of T4 cells is low, the total T-cell count is lower than normal, and the T4/T8 cell ratio is lower than the usual 2 to 1 ratio.

HIV infection not only reduces the number of T4 cells, it can also impair a T4 cell's functioning. HIV-infected individuals with very low T4 cell counts tend to have more serious infections. Therefore, regular immunologic tests to determine T4, T8, and total T-cell counts can be an important element in monitoring the health of an HIV-infected person.

Two types of HIV have been identified to date: HIV-1 and HIV-2. HIV-1 is the predominant HIV type in the United States and throughout the world. HIV-2 is primarily found in West Africa.

Origin of HIV Virus
HIV-1 originated in non-human primates, probably chimpanzees.
The origin of HIV-2 has been identified as being another monkey species, the sooty mangabey (Cercocebus atys).