Figure 1

Direct and indirect activation mechanisms. With the direct activation mechanism the donor’s Antigen Presenting Cell, expressing its MHC molecule (red) and carrying a general epitope (green), is recognized from the T-Cell Receptor and activates T-cell clonal expansion. In this case it is the actual MHC molecule which is recognized as non-self from the TCR. In the lower part of the figure, the two possible indirect activation scenarios: self APC, expressing self MHCs, can present an epitope from graft antigens recognized as non-self (blue) or from donor’s MHC (red).