In the case of transplantation, MHC genes and molecules from the donor serve as the triggers of rejection, and the MHC and genes in the recipient determine his/her capacity to respond to allogeneic antigens, in the same way they do it for conventional immune responses. In other words, it appears that it is the combination and the “interplay” between the donor and the recipient MHC genes, not only the donor MHC antigens, what determines immunologically the outcome of the transplant in the recipient. The MHC genes are the most polymorphic genes in the genome and each genetic variant, or allele, in the donor provides an array of antigenic possibilities to recipients. But the immunogenicity of these allleles for recipient T and B cells depends on the particular alleles present in the recipient. Thus, certain recipient individuals will produce alloantibodies to certain donor alleles but no others. This is due to the fact that certain conformational antigenic determinants on allogeneic MHC molecules are present also on certain MHC alleles in the recipient (shared determinants), not eliciting an immune response, while others not shared,becoming immunogenic.