The CD4 gene encodes the T4 antigen, a glycoprotein of approximately 55 kDa, expressed on the surface of helper T lymphocytes.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 GP120.