The following explanation is greatly simplified and is by no means complete or definitive. The exact mechanisms of how infection by the Human Immunodeficiency Virus (HIV) leads to Acquired Immune Deficiency Syndrome (AIDS) is not completely understood.
CD4+ T lymphocytes are important cells within the immune system. They have a pivotal role in the immune response and they interact with macrophages, other T cells, B cells and natural killer cells. They can interact directly or via messenger molecules known as cytokines.
In the body, the way in which viral infections are combated is for infected cells to present viral proteins on their cell surfaces and for cytotoxic cells such as CD8+ T lymphocytes to lyse these cells.
In HIV infection, it is the CD4+ T cells which are infected and they present viral proteins and then are lysed. It seems that the CD8+ T cell response is not sufficient to totally clear HIV infection as the HIV virus evades recognition by remaining latent for years, as well as by other methods such as antigenic shift. As more and more CD4+ T cells are destroyed, immune function falls and opportunistic infections attack.
Clinicians use CD4+ T cell levels as a marker for the progression of HIV disease.
STAGES OF HIV/AIDS|
CD4+ T CELL COUNT|
< 500 x 10 000 000/l
< 200 x 10 000 000/l
< 50 x 10 000 000/l
As well as using CD4+ T cell count, clinicians also use the presence of indicator diseases as a definition of having AIDS. Indicator diseases are the ones commonly seen in HIV patients such as Candidiasis, Cryptococcosis, Herpes simplex virus, Tuberculosis, Pneumocystis carinii, Toxoplasmosis etc.