In the course of studies aimed at determining whether MDP was antigenic or not, a hitherto unreported phenomenon was noticed. Injection (a provocative injection) of muramyl dipeptide (MDP) caused severe inflammation, with hemorrhage and necrosis in the footpads of guinea pigs, where tubercle bacilli in water-oil emulsion (a preparatory injection) had been injected 3-8 wk earlier. Sometimes the reaction was accompanied by generalized and fatal shock. Several related substances were tested, and only a combination of tubercle bacilli, or MDP plus proteins as the preparatory injection, and MDP as the provocative injection was found to induce this inflammatory necrotic reaction. Development of delayed hypersensitivity to protein antigens may be important for priming, but MDP and not the protein antigens provoked the reaction. This reaction was, so far, only observed in guinea pigs. Although this reaction appears to be similar to the Shwartzman reaction, the two reactions were found to differ from each other in several important points.

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