CD4 antigen

In medicine, the CD4 antigens are "55-kda antigens found on helper-inducer T-lymphocytes and on a variety of other immune cell types. CD4 antigens are members of the immunoglobulin supergene family and are implicated as associative recognition elements in major histocompatibility complex class II-restricted immune responses. On T-lymphocytes they define the helper/inducer subset. CD4 antigens also serve as interleukin-15 receptors and bind to the HIV receptors, binding directly to the HIV envelope protein G120."

The number of CD4-positive T-lymphocytes is called the CD4 lymphocyte count and it is reduced in Human Immunodeficiency Virus Type 1  and AIDS.