Transplants between inbred strains fail second set rejection is faster, more intense (due to immunologic memory) second set from an unrelated part is akin to a first set (specificity) memory resides in lymphocytes (and can thus be transferred) Terminology: autologous same person syngeneic between genetically identical individuals (inbred strains) allogeneic genetically different but same species xenogeneic different species. Genes responsible for transplant rejection are at hla and encode mhc i and ii. % graft survival = (3/4)n where n=number of histocompatability genes (approx. Cd4 cell involvement in alloresponse: they recognize foreign mhcii molecules. Il4/5/6: activate b cells, stimulate antibody production causes vascular damage in vivo. Cd8 cells recognize and target foreign alloresponsive cells based on mhci evidence: If a 3rd party cell has same mhci as stimulator of alloresponse it will also be targeted for cytolysis. Covering stimulator cell mhci with antibodies protects them. Transfecting mhci genes to previously resistant target cells renders them susceptible to cytolysis.