As alluded to before, transplant rejection is the most powerful immune response with peculiar features different from conventional immune responses. Even in the absence of prior alloimmunisations, through the phenomenon of cross reaction, circulating alloreactive memory T cells are present and detectable. The T cell reseptor has the inherent capacity to bind MHC molecules and easily cross-react with allogeneic MHC; peptide complexes, whose stucture resembless that of the complexes formed by self MHC molecules and any given environtmental antigen. Thus, memory T cells originally specific for different infectious organisms can attact rapidly and efficiently the transplant tissue as if the patient would have been transplanted with those alloantigens before. Added to this issue, memory T cells specific for alloantigens can be formed and boosted in recipients having history of blood transfusion, multiple pregnancies and prior transplant.