The retest contact hypersensitivity reaction in guinea pig skin and oral mucosa.

A chemical sensitiser, 2:4 di-nitro-chloro-benzene was reapplied to previously elicited delayed hypersensitivity sites in guinea pig skin and oral mucosa and the macroscopic and microscopic appearances of these “restest” sites compared with adjacent delayed hypersensitivity reactions to the chemical produced at the same time. In the sites where the chemical sensitiser had been reapplied, oedema was evident, macroscopically, at an earlier stage than in the delayed hypersensitivity sites. Microscopically, vascular dilatation and the presence of a mononuclear infiltrate were apparent in the “retest” sites earlier than in the delayed hypersensitivity sites. There was an initial reduction in the count of eosinophil polymorphonuclear leucocytes in the “retest” and delayed reaction in skin and oral mucosa. A late rise in the count of these cells occurred in the “retest” reactions in skin and oral mucosa, but was more pronounced in the mucosa. There was no significant alteration in the mast cell count in the “retest” or delayed hypersensitivity reactions in the skin or oral mucosa. It is suggested that the mononuclear infiltrate remaining after the delayed hypersensitivity reaction is associated with the more rapid vascular changes which accompany the “retest” reaction.