The RhLA complex of rhesus monkeys. XI: Positive MLC reactions between identical sibs caused by alloimmunization.

D-locus identical pairs of monkeys were alloimmunized. An exchange of skin and blood between initially MLC-negative unrelated monkeys frequently led to strong mutual reactivity in mixed cultures. This was not the case when RhLA identical siblings were similarly (mutually) immunized. These data led to the postulation of a second RhLA-linked MLC locus, named D', which exerts its influence only after alloimmunization. Interestingly, strong MLC responsiveness between the RhLA identical siblings could be provoked after further immunization with tissue from a thired party animal. This unexpected MLC-reactivity is ascribed to the in fluence of minor non MHC linked loci which express themselves if the responder has been sensitized against a disparate D'-product. It is suggested that the postulated D' locus has a "helper function" in that it assists in triggering proliferative responses in mixed cultures against the products of these non MHC loci.