In vivo macrophage-directed lymphocyte traffic requires macrophage activation

Lymphocytes are attracted to activated macrophages in tissue. In the absence of exogenous activators, only syngeneic macrophages can become sufficiently activated, to produce lymphocyte attractants. Allogeneic macrophages require exogenous activation to accomplish lymphocyte recruitment. It is likely that graft rejection is initiated by a nonspecific inflammatory reaction within the graft. Once host macrophages have entered the graft they may be activated by the inflammatory process to produce lymphocyte attractants. As lymphocytes are called to the graft, those recognizing donor determinants may expand to effect graft loss.