Delayed onset erythematous skin reactions elicited in guinea pigs early in the course of sensitization with azobenzenearsonate-protein conjugates or with protein antigens in incomplete Freund's adjuvant or in saline were found to have a characteristic morphology which sets them apart from delayed hypersensitivity and the classic antibody mediated reactions. The principle feature was massive dermal infiltration with basophilic leukocytes. Mononuclear cells of several types including activated and small lymphocytes, monocytes, macrophages, and blast cells were also present. Such reactions have in the past been designated Jones-Mote hypersensitivity, but we prefer the descriptive term cutaneous basophil hypersensitivity (CBH) for the reasons given.

Occasional basophils extruded their granules, and individual granules, retaining their characteristic ultrastructure, were commonly seen in the interstitium. However, intercellular junctions between endothelial cells were closed except during cell emigration and there was no morphologic evidence of an histamine-like effect. The majority of basophils, moreover, did not degranulate but underwent nuclear pyknosis and cytoplasmic degeneration and were phagocytosed by macrophages. Phagocytosed basophil granules retained their ultrastructure.

Skin tests performed at late intervals after sensitization had a different time course and morphology. Animals sensitized with protein antigens in complete Freund's adjuvant developed delayed hypersensitivity; however, reactions elicited in such animals at early (but not late) intervals after sensitization contained a prominent basophil component. We interpret such reactions to be a mixture of delayed hypersensitivity and cutaneous basophil hypersensitivity. The function of the basophil in CBH and its relation to the mononuclear cells which accompany it are unknown, and various possibilities are discussed.

We conclude that cutaneous basophil hypersensitivity is a distinct immunologic and morphologic entity, occurring early in the course of sensitization with protein antigens incorporated in any of several vehicles. The mechanism of the reaction is presently unknown, and a general hypothesis to explain its pathogenesis has been proposed.

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