CD4 cells are white blood cells that help fight infection and thus play an important role in the function of the body's immune system. These are produced in the thymus gland and they circulate via the blood and lymphatic system.
This test measures the number of CD4 cells in the blood and is a useful aid to establish the level of immune suppression especially in conjunction with testing for HIV.
CD4 cells are also called T-helper cells as they assist recognising and killing of foreign "material" e.g. bacteria or viruses. HIV virus is able to enter these cells but not be destroyed and as the infection progresses, the number of CD4 cell decrease. On the other hand, if the infection is treated and the viral load is undetectable, a slow rise in cells are seen as the immune function is restored. Patients with a very low CD4 count are vulnerable to certain infections not seen in healthy individuals - these are called opportunistic infections.
The CD4 count is reported as an absolute level or count of cells and normally ranges between 500 - 1,200 cells/mm3 in adults. In children the results are sometimes expressed as a percentage of total lymphocytes.
CD4 count testing is usually performed with a HIV viral load after the diagnosis of HIV was made or when a patient is started on treatment and followed up at regular intervals.