Picryl Chloride hypersensitization of guinea pigs produced delayed skin reactions to this drug. In the whole blood of such animals leukocyte agglutination occurred upon addition of a picryl-horse serum conjugate in vitro but not with picryl chloride nor horse serum alone. The substance responsible for this agglutination existed in serum as a nondialyzable, heat-labile constituent. Because of the parallel of leukoagglutinating activity of the sera to the ability to induce passive cutaneous anaphylaxis, the leukoagglutination phenomenon is believed more closely related to the immediate than the delayed form of hypersensitive response.

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