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A bacteriotropic principle contained in the normal serum, which is greatly increased by any of the intestinal infectious bacterial diseases and can also be produced artificially by the injection of non-fatal doses of such bacteria as Bacillus typhosus and Spirillum cholerae into animals. The serum of such animals will agglutinate the organism used for injection in very high dilution, the agglutinin combining with the receptors of the bacterial cell. It is destroyed by 75 C., acids and alkalis, and consists of two groups, a combining and an agglutinating group.