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 gp120.