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. Forms disulfide-linked homodimers at the cell surface. Interacts with LCK. Interacts with PTK2/FAK1. Binds to P4HB/PDI. Interacts with IL16; this interaction induces a CD4-dependent signaling in lymphocytes. 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. 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 plays an important role for CD4 internalization. Mice lacking Cd4 display markedly decreased T-helper cell activity.