What does the T-cell receptor recognize when it docks on an MHC-encoded restricting element?

The postulate is analyzed that single V-gene segments encode recognition of the allele-specific determinants (a) required for the restrictive response of the alphabeta TCR to peptide. The consequence of this is that the positively selected V-domain, Valpha or Vbeta, engages an allele-specific determinant (a) on one subunit or domain of the MHC-encoded restricting element. The entrained V-domain docks on an invariant determinant (i) on the complementing subunit or domain. Consequently, each functional V-domain expresses an anti-a site and an anti-i site, and all subunits or domains of MHC-encoded restricting elements express an a- and i-determinant. The evidence, both biological and structural, discussed here strongly supports this postulate which has far reaching consequences.