Phytohemagglutinin (PHA) skin test. Characterization of immunological properties and clinical application

Immunological properties of phytohemagglutinin (PHA) skin reaction were investigated by animal and clinical experiments. In the guinea pigs an intradermal dose of PHA-P produced erythema and induration with a maximal response at 24 hours after the injection. Histologically it was characterized by perivascular infiltration of lymphoid cells in the dermis and subcutis, being similar to that of tuberculin (PPD) skin reaction. PHA skin reaction, however, showed some difference from that of PPD in the initial cellular response in that the former was composed of small mononuclear cells and granulocytes with rapid development and the latter was composed of large mononuclear cells (macrophages) and granulocytes with slow development. Intradermal injection of 1:1000 dilution of PHA-P produced a similar erythema in man. In 39 of 59 patients with connective tissue diseases, the results of the in vivo (skin test) and in vitro (lymphocyte transformation) response to PHA correlated well. In the 59 patients, the incidence of the positive rate of the PHA tests (55.9%) was significantly higher than that of the DNCB test (33.9%) and of the PPD test (23.7%). These observations suggest that the PHA skin test has properties of delayed hypersensitivity and is highly sensitive and that it may be a useful measure of cell-mediated immunity.