In graft rejection, T-cell stimulation by donor APCs and self-APCs (present
ing peptides of donor origin) has been called 'direct' and 'indirect' recog
nition, respectively. Here, Dina Gould and Hugh Auchincloss consider the tr
aditional arguments favoring direct recognition and highlight recent findin
gs suggesting the importance of indirect responses, thereby questioning som
e of our basic concepts of transplantation immunology.