Structure of Antigen-Recognizing Receptor (TCR) and Coreceptor CD4

An antigen-recognizing T-cell Receptor (TCR) may be composed of α chain and β chain (type αβTCR), and γ chain and δ chain (type γδTCR). αβTCR-T cells originate from the thymus, whereas γδTCR-T cells (γδT cells) are differentiated in the thymus and outside the thymus. Each chain has a variable and constant domain. αβTCR reacts only to peptide antigens, whereas γδTCR can react to phospolipid antigens (phosphoantigens) probably presented by CD1.
CD3, an accessory molecule, is associated with TCR and required for signaling and re-expressing TCR. The CD3 consists of 4 dimeric parts, εδ, εγ, ζζ, and ζη.
Compared to TCR, which senses "non-self," coreceptor CD4 recognizes "self," namely Class II HLA molecules.