Phytohemagglutinin induces major short-term protease-sensitive lymphocyte traffic involving high endothelium venule-like blood vessels in acute delayed-type hypersensitivity-like reactions in skin and other tissues.

Injection of phytohemagglutinin induces dose-dependent acute delayed-type hypersensitivity-like reactions in young pig skin which attract large numbers of labeled peripheral blood lymphocytes and lymphoblasts but not red cells. Similar reactions are induced in the gut wall, joints, and draining lymph nodes, and also in the skin of sheep and cattle, and by concanavalin A. Labeled peripheral blood lymphocyte entry starts within 2 h, is maximal at 6-18 h and is effectively over by 48 h and, both early and late, is markedly inhibited in a selective way by trypsinization, like high endothelium venule-mediated entry to lymphoid tissues. Electron microscopy showed development of high endothelium venule-like blood vessels with intramural lymphoid cells in mononuclear infiltrates. Inductive processes in this model short-term peripheral defense mechanism are being investigated.