T-cell surface glycoprotein CD4 (CD4)

Integral membrane glycoprotein which plays a vital role in the immune response and serves multiple functions in responses against both internal and external offenses. The antigens presented by class II peptides are derived from extracellular proteins while class I peptides are derived from cytosolic proteins.

Interacts concurrently with the T-cell receptor (TCR) and the MHC class II presented by antigen presenting cells (APCs). Consequently, recruits the Src kinase LCK to the vicinity of the TCR-CD3 complicated.