Tc cells may be Syngeneic: Generated in response to self (autologous) cells modified by viral infection or other foreign proteins Allogeneic: Generated in response to cells that express foreign MHC products (eg, in organ transplantation when the donor's MHC molecules differ from the recipient's) Some Tc cells can directly recognize foreign MHC (direct pathway); others may recognize fragments of foreign MHC presented by self MHC molecules of the transplant recipient (indirect pathway).