T-cell receptors recognize, directly, allogeneic MHC molecules expressed on donor cells. Also, allogeneic MHC may be processed and presented as an exogenous foreign protein. As a consequence, up to 2% of T-cells may respond as compared to the clonal T-cell response to a specific peptide presented in context with self-MHC. Thus, the fewer the MHC differences between the donor and recipient the less severe the tissue rejection.
Immunosupressive drugs, such as cyclosporin A ( reduces T-cell activation, both T-helper cells and CTL activity ), are used to reduce the vigor of the immune response to a tissue graft. However, this immunocompromised state renders the recipient susceptible to virus-related diseases such as B-cell lymphomas (Epstein-Barr virus), squamous cell carcinomas (human papilloma virus), and Kaposi's sarcoma (a herpes virus) as well as viral infections (cytomegalovirus); also graft-versus-host disease caused by alloreactive T-cells within the donor tissue that can cause tissue damage in the recipient.