CD4 is a glycoprotein that functions as a co-receptor for the T-cell receptor (TCR) and is primarily expressed on the surface of helper T cells, monocytes, macrophages, and dendritic cells. It is a member of the immunoglobulin superfamily, consisting of four immunoglobulin domains (D1 to D4) that interact with MHC class II molecules on antigen-presenting cells. This interaction, mediated by the D1 domain of CD4, ensures that T cells recognize antigens presented by MHC II. The cytoplasmic tail of CD4 contains motifs that recruit the tyrosine kinase Lck, which amplifies T cell activation signals. Additionally, CD4 is crucial in the immune response, as it helps activate CD4+ T helper cells, which in turn stimulate other immune cells to combat infections. However, CD4 is also the primary receptor for HIV, allowing the virus to enter host T cells and cause immunodeficiency.