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Figure 7-2: Cartoon of the human T cell receptor and its subunits. Part A shows subunit composition of the human T cell receptor. The TCR subunits are held together by S-S bonds and are closely associated with either the CD4 or CD8 molecule and chains of the CD3 complex. The subunits are anchored in the cell membrane. The CD3 complex consists of three subunits referred to as gamma, delta, and epsilon. Associated in the TCR complex is another pair of 16 kD homodimer (32 kD nonreduced), subunits existing as homodimers of zeta or heterodimers of zeta and eta. Part B shows the structure of the Ti subunits. The predicted primary structure of the chain subunit after translation from the cDNA sequence is depicted, as are the variable region leader (L), V, D, and J segments, a hydrophobic transmembrane segment (TM) and cytoplasmic part (Cyt) in the C region, potential intrachain sulfhydryl bonds (S S), and the single SH group (S) that can form a sulfhydryl bondwith the subunit. Part C shows a scheme of the genomic organization of human and chain genes. In the locus, V indicates the V gene pool located 5′, at an unknown distance from the D 1 element, the J 1 cluster, and the C 1 constant region gene. Further downstream, a second D 2 element, J 2 cluster, and C 2 constant region gene are indicated. A similar nomenclature is used for the Ti locus, in which only a single constant region is found. ?D indicates the uncertainly about the existence of a putative Ti diversity element. (From Reference 1).