Grafted kidney may be rejected in a patient due to:

The T-lymphocytes mediate the cell-mediated immunity (CMI). T-cells respond to antigens by producing a clone of T-cells. Each T-cells recognizes a specific antigen. Therefore, there are different types of T-cells for every antigen which the body encounters. CMI is involved in immunity against infectious diseases, transplantation immunity, and graft versus host reaction. Cytotoxic T-cells are mainly responsible for graft rejection.