Antigen, CD4

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.