This test measures CD4 cell concentrations in the blood. CD4 is a type of lymphocyte (white blood cells). They are an important part of the immune system. CD4 cells are sometimes called T-cells. There are two main types of T-cells. T-4 cells, also called CD4, are “helper” cells. They lead the attack against infections. T-8 cells (CD8) are “suppressor” cells that end the immune response. CD8 cells can also be “killer” cells that kill cancer cells and cells infected with a virus.
CD4 T lymphocytes (CD4 cells) help coordinate the immune response by stimulating other immune cells, such as macrophages, B lymphocytes (B cells), and CD8 T lymphocytes (CD8 cells), to fight infection. They are considered to be the "directors of the immune orchestra." Some infectious diseases like HIV infection can deplete CD4 cells. CD4 cells under 200 cells/mm3 are considered low and classify an HIV-infected person as having AIDS.
Some people not infected with HIV may have low CD4 cell counts due to other autoimmune conditions or negative lifestyle factors.