T cells are a type of white blood cells called lymphocytes. They make up part of the immune system. T cells help the body fight diseases or harmful substances.

CD4 cells are a type of T cell. T cells are cells of the immune system. They are also called "helper cells." Acute HIV infection progresses over time (usually a few weeks to months) to asymptomatic HIV infection (no symptoms) and then to early symptomatic HIV infection. Later, it progresses to AIDS (advanced HIV infection with CD4 T-cell count below 200 cells/mm3 ). The CD4 count serves as the major laboratory indicator of immune function in patients who have HIV infection.