Cluster of antigens , with which antibodies react, that characterize a cell surface marker .

Lymphocytes can be divided into subsets either by their functions or by surface markers. The availability of monoclonal Abs raised against lymphocytes has allowed for the demonstration of several lymphocyte subsets, which express a combination of certain molecules on their surfaces. These surface markers have been designated clusters of differentiation (CD). Already, 78 CDs have been identified as well as the monoclonal Abs used to define them, their molecular weights, and cellular distribution. For example, CD23 , the receptor for the FC portion of IgE and CD8 (T8), a protein embedded in the cell surface of suppressor T-lymphocytes.

CD4

One of the most infamous CDs due to its importance in AIDS. CD4 (T4) is the protein embedded on the surface of T-helper and other white-blood cells to which HIV attaches itself. Also found to a lesser degree on the surface of monocytes / macrophages , Langerhans cells , astrocytes, keratinocytes and glial cells. HIV invades cells by attaching itself to the CD4 molecule (CD4 receptor). The number of T4 cells in a blood sample is used to measure the health of the immune system in people with HIV.