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Characteristics of the allergic skin reaction to free carrier protein in sensitized guinea pig with hapten-carrier conjugate have been studied. The animals sensitized with a low dose (4 micrograms) of dinitrophenylated bovine gamma globulin with Freund's complete adjuvant demonstrated a dermatitis when challenged intradermally with a small dose (10 micrograms) of bovine gamma globulin on the 10th or 14th day. The dermatitis was grossly and histologically similar to the classical form of delayed-type hypersensitivity skin reaction except lesser induration and less abundant neutrophil infiltration in the former. This reaction was clearly different from cutaneous basophil hypersensitivity because of the long lasting time course and fewer basophil infiltration in the former. No humoral antibody to carrier protein was detected in the sera of the sensitized animals by passive cutaneous anaphylaxis or by immunodiffusion in agar plates, and the state of the hypersensitivity could be passively transferred with peritoneal exudate cells and spleen cells. These observations indicated that the skin reaction to carrier protein might be of primarily delayed-type hypersensitivity. The vascular permeability change at the reaction sites was observed as a single phase delayed response and preceded the maximum erythema by about ten hours.

Rabbit antisera (APS) against normal guinea pig peritoneal exudate polymorphonuclear leukocytes (PMN), when injected intraperitoneally three times within 24 h into guinea pigs, lead to a marked fall in the blood PMN count (below 100/mm3) 24-72 h after the injection and a mild depression in the number of circulating mononuclear leukocytes (MNC) at 36-72 h. APS treatment of guinea pigs led to a marked suppression of delayed hypersensitivity skin reactions in response to dinitrophenylated bovine gamma-globulin. The volume, induration, mean diameter of the reaction and leukocyte (both PMN and MNC) emigration into local sites were suppressed, as was, to a lesser extent, vascular permeability. Based on these observations it appears that circulating PMN are required for the full expression of delayed hypersensitivity reaction engendered by bovine gamma-globulin.