Integral membrane glycoprotein that plays an essential role in the immune response and serves multiple functions in responses against both external and internal offenses. In T-cells, functions primarily as a coreceptor for MHC class II molecule:peptide complex. The antigens presented by class II peptides are derived from extracellular proteins while class I peptides are derived from cytosolic proteins. Interacts simultaneously with the T-cell receptor (TCR) and the MHC class II presented by antigen presenting cells (APCs). In turn, recruits the Src kinase LCK to the vicinity of the TCR-CD3 complex. LCK then initiates different intracellular signaling pathways by phosphorylating various substrates ultimately leading to lymphokine production, motility, adhesion and activation of T-helper cells. In other cells such as macrophages or NK cells, plays a role in differentiation/activation, cytokine expression and cell migration in a TCR/LCK-independent pathway. Participates in the development of T-helper cells in the thymus and triggers the differentiation of monocytes into functional mature macrophages. (Microbial infection) Primary receptor for human immunodeficiency virus-1 (HIV-1) (PubMed:2214026, PubMed:16331979, PubMed:9641677, PubMed:12089508). Down-regulated by HIV-1 Vpu (PubMed:17346169). Acts as a receptor for Human Herpes virus 7/HHV-7 (PubMed:7909607). Forms disulfide-linked homodimers at the cell surface. Interacts with LCK (PubMed:16888650). Interacts with PTK2/FAK1 (PubMed:18078954). Binds to P4HB/PDI. Interacts with IL16; this interaction induces a CD4-dependent signaling in lymphocytes (PubMed:1673145). Interacts (via Ig-like V-type domain) with MHCII alpha chain (via alpha-2 domain) and beta chain (via beta-2 domain); this interaction increases the affinity of TCR for peptide-MHCII. CD4 oligomerization via Ig-like C2-type 2 and 3 domains appears to be required for stable binding to MHCII and adhesion between T cells and APCs (PubMed:27114505, PubMed:21900604, PubMed:7604010). (Microbial infection) Interacts with HIV-1 Envelope polyprotein gp160 and protein Vpu (PubMed:2214026, PubMed:16331979, PubMed:9641677, PubMed:7604010). (Microbial infection) Interacts with Human Herpes virus 7 surface proteins. Localizes to lipid rafts (PubMed:12517957, PubMed:9168119). Removed from plasma membrane by HIV-1 Nef protein that increases clathrin-dependent endocytosis of this antigen to target it to lysosomal degradation. Cell surface expression is also down-modulated by HIV-1 Envelope polyprotein gp160 that interacts with, and sequesters CD4 in the endoplasmic reticulum. Highly expressed in T-helper cells. The presence of CD4 is a hallmark of T-helper cells which are specialized in the activation and growth of cytotoxic T-cells, regulation of B cells, or activation of phagocytes. CD4 is also present in other immune cells such as macrophages, dendritic cells or NK cells. The Ig-like V-type domain mediates the interaction with MHCII. Palmitoylation and association with LCK contribute to the enrichment of CD4 in lipid rafts. Phosphorylated by PKC; phosphorylation at Ser-433 plays an important role for CD4 internalization. The OKT monoclonal antibodies are widely used for the analysis of human peripheral blood T-lymphocytes. OKT4 reacts with T-helper/inducer lymphocytes. The OKT4 epitope of the CD4 cell-surface protein is polymorphic in white, black, and Japanese populations. The variable phenotypic expression is due a CD4 polymorphism. OKT4 positive individuals carry Arg-265 and OKT4 negative individuals carry Trp-265 [MIM:613949]. The disease is caused by variants affecting the gene represented in this entry. CD4 entry