The value of an assessment of erythema and increase in thickness of the skin reaction for a full appreciation of the nature of delayed hypersensitivity in the guinea pig.

Delayed type skin reactions in guinea pigs have been assessed by measuring three parameters: increase in skin thickness, diameter of erythema and intensity of erythema. Some groups of animals were immunized with different protein antigens in Freund's complete adjuvant with or without cyclophosphamide (CY) pretreatment; others received a single high dose of antigen intravenously at the time of immunization. The results emphasize the importance of measuring all three parameters for several days after skin testing. The intensity or erythema was found to be an especially useful parameter for assessing CY-induced modification of delayed hypersensitivity (DH) reactions. The increase in skin thickness, which can be measured objectively, was also valuable as both immediate and DH reactions are characterized by induration. The diameter of erythema could only be measured accurately for a short time after skin testing. Furthermore, the effects of intravenous antigen and CY pretreatment were not reflected by that parameter.