CD4 encodes the membrane glycoprotein of T lymphocytes. CD4 antigen acts as a co receptor with T cell receptors on T lymphocytes to recognize antigens displayed by antigen-presenting cells in the context of class II MHC molecules. CD4 antigen is also the main receptor that human immunodeficiency virus enters through interaction with HIV Env gp120 subunit. This gene is not only expressed in T lymphocytes, but also in B cells, macrophages, granulocytes, and various regions of the brain. The function of this protein is to initiate or enhance the early stages of T cell activation and may play an important role as an indirect mediator of neuronal damage in infectious and immune-mediated diseases of the central nervous system. Multiple alternative splicing transcript variants encoding different subtypes have been identified in this gene.