CD4 §

A single-chain protein composed of four Ig-like domains that acts as a co-receptor to activate CD4+ t-cell, whilst the TCR is contacting a pMHC. D1 and D2 are packed tightly together to form a rigid rod. The MHC-binding region is found on the lateral face of the D1 domain, and binds to a hydrophobic crevice of the α₂-β₂ junction.

The intracellular portion of CD4 binds a cytoplasmic tyrosine kinase called Lck, which in proximity of the TCR helps activate the signalling cascade induced by antigen recognition.