Allograft rejection may involve which of the following?

A. Helper T cells.

C. Cytotoxicity.

D. Cytokines.

DISCUSSION: The effector mechanisms that underlie the pathogenesis of allograft rejection remain the subject of controversy. Rejection, like delayed-type hypersensitivity, may be mediated by helper T cells, which release cytokines that activate other cells such as macrophages and directly alter endothelial cell functions. On the other hand, rejection may be mediated by cytotoxic T cells, which kill or injure target cells through direct interactions. Both types of cells can be found in grafts undergoing rejection, and there is experimental evidence suggesting that both may be involved. Veto cells kill T cells, which recognize MHC antigens on the veto cell surface; this action is thought to contribute to tolerance and not to rejection. The Arthus reaction is an immune response that, in contrast to allograft rejection, is mediated primarily by antibodies and not by cells.