Experimental Intradermal Granuloma Formation**From the Department of Pathology, Beth Israel Hospital, Harvard Medical School, and the Division of Tumor Immunology, Children's Cancer Research Foundation, Boston, Massachusetts. (Reprint requeats to Dr. E. J. Kasdon, Department of Pathology, Beth Israel Hospital, 330 Brookline Avenue, Boston, Mass. 02215.)

Experimental induction of an intradermal granulomatous hypersensitivity reaction in the primarily immunized guinea pig was studied by means of tight microscopy during the course of the immune response. Animals were sensitized to dinitrophenylated human serum albumin (DNP10-HSA), and challenged with this antigen covalently linked to Sepharose 2B beads by the cyanogen bromide reaction. At appropriate intervals, the dermal changes produced by this complex antigen were compared with those produced by DNP10-HSA or soluble antigen. Sepharose 2B beads atone, and DNP10-HSA plus Sepharose 2B in the same animal. The complex antigen produced lesions characteristic of granulomatous hypersensitivity 5 and 11 days beyond the time of the skin test. The others showed the characteristic responses of hypersensitivity, foreign-body reaction, and a mixture of delayed-hypersensitivity and foreign-body reaction, but the intensity of the reactions had diminished by 5 days and had disappeared after 11 days. These results indicate that a dermal granulomatous hypersensitivity reaction may be differentiated from a foreign-body reaction. They also suggest that locally retained antigen interacts with sensitized lymphocytes to contribute to granuloma formation