Fig. 6. Components of the T-cell receptor (TCR) complex. The γ, δ, and ε chains are present as monomers, noncovalently associated with the T-cell receptor αβ heterodimer and with one another. Þgz and Þgh are present as ÞgzÞgz homodimers or as ÞgzÞgh heterodimers, and these chains also may physically associate with the TCR or with other CD3 chains. Disulfide-bonded loops of immunoglobulin (Ig)-like domains are indicated in the extracellular regions of the TCR α and β chains and the CD3 γ, δ, and ε chains. The + and - symbols refer to charged residues in the transmembrane regions that probably mediate association of the chains. Antigen receptor activation motifs in cytoplasmic tails are conserved sequences that include sites of tyrosine phosphorylation. (After Abbas AK, Lichtman AH, Pober JS. Cellular and Molecular Immunology, p 145. 2nd ed. Philadelphia: WB Saunders, 1994.)