CD4 is a glycoprotein that serves as a co-receptor for the T-cell receptor (TCR) and is found on the surface of immune cells such as helper T cells, monocytes, macrophages, and dendritic cells. It is a member of the immunoglobulin superfamily and has four immunoglobulin domains (D1 to D4) that are exposed on the extracellular surface of the cell. The D1 domain of CD4 interacts with the β2-domain of MHC class II molecules, allowing T cells displaying CD4 to be specific for antigens presented by MHC II. The cytoplasmic tail of CD4 contains a sequence of amino acids that allow it to recruit and interact with the tyrosine kinase Lck, which is crucial for T cell activation. CD4 is also the receptor for HIV, which uses the protein to gain entry into host T-cells through its viral envelope protein gp120.