The cutaneous, ophthalmic, and systemic reactions of normal rabbits to Gram-negative bacterial endotoxins have been compared with the classical reactions of bacterial hypersensitivity, and in each case certain similarities have been found. It has also been shown that the Shwartzman phenomenon can be reproduced with tuberculin, in BCG-vaccinated rabbits, and with suspensions or extracts of heat-killed Group A streptococci in rabbits previously sensitized to these bacteria.

These considerations suggest the hypothesis that the biologic activity of endotoxins may be based on the existence in "normal" animals of delayed or tuberculin-type hypersensitivity to these materials.

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