This gene encodes the CD4 membrane glycoprotein of T lymphocytes The CD4 antigen acts as a coreceptor with the T-cell receptor on the T lymphocyte to recognize antigens displayed by an antigen presenting cell in the context of class II MHC molecules The CD4 antigen is also a primary receptor for entry of the human immunodeficiency virus through interactions with the HIV Env gp120 subunit This gene is expressed not only in T lymphocytes but also in B cells macrophages granulocytes as well as in various regions of the brain The protein functions to initiate or augment the early phase of T-cell activation and may function as an important mediator of indirect neuronal damage in infectious and immune-mediated diseases of the central nervous system Multiple alternatively spliced transcript variants encoding different isoforms have been identified in this gene