Histopathology of delayed-onset hypersensitivities in contact-sensitive guinea pigs

We have studied the specificity of macroscopic responses and the corresponding cellular infiltrates in Giemsa-stained, glycol methacrylate embedded tissue sections of delayed skin reactions elicited by contact sensitizers, homologous and heterologous hapten-protein conjugates, and tuberculin (PPD) in guinea pigs. The animals were sensitized with contact sensitizers emulsified in Freund's complete adjuvant and tested at 19 days and, after repeated contact testing with homologous sensitizer, at 48 days after sensitization. At both times, all delayed reactions contained numerous mononuclear cells. The reactions elicited by contact sensitizer or hapten-protein conjugate also contained significant numbers of basophils which were lacking in PPD reactions. Contact reactions were highly specific macroscopically and microscopically while reactions elicited by conjugates displayed considerable macroscopic and microscopic cross-reactivity. The greater cross-reactivity of conjugate-elicited reactions compared to contact reactions may reflect differences in mechanism of initiation or mediation of these two reactions.