Exposure to another person’s HLA antigens may also induce alloactivation of T lymphocytes. Two general pathways of T-cell alloactivation have been recognized in transplant immunity.
The direct pathway refers to the alloreactive responses of recipient T cells to donor APC expressing incompatible HLA antigens. It provides a powerful mechanism of T-cell alloactivations.
In the indirect pathway, allogenic HLA antigens are released from donor cells, then taken up and processed (through the exogenous pathway) by recipient APC, and presented to recipient T cells as allopeptides bound to self-HLA molecules