Transplantation, ABO incompatibility and immunology

An allograft is tissue transplanted from another individual within the same species. Mechanical trauma to a graft and recipient transplant site along with graft-derived proinflammatory mediators stimulate a non-specific innate immune response. Dendritic cells and macrophages present foreign antigen to the adaptive immune system cells and thus initiate a specific and directed response. In order to respond to a specific pathogen, an individual must be able to recognize foreign cells as non-self. Major and minor histocompatibility antigens (MHCs) and the ABO blood group antigens are central to distinguishing one human from another and therefore in recognizing self from non-self. Genetic polymorphism describes genes encoded by varying alleles resulting in varied phenotypes within a species. The blood group and MHC are polymorphic, with many different possible allelic combinations leading to differences between individuals and allowing an individual to recognize self from non-self. Rejection describes the graft injury and loss of function due to the recipient’s non-acceptance of the graft as ‘self’ and the response which aims to remove it from the body. Rejection can be classified into hyperacute, acute and chronic states. Both cell-mediated and antibody-mediated mechanisms lead to allograft tissue destruction. By minimizing MHC mismatch and using immunosuppression therapy, the immune response to a graft can be reduced. This involves familial grafting when possible, matching donors and recipients for similar human leucocyte antigen and identification of preformed recipient antibodies.