The manner in which a single injection of S. typhosa endotoxin effects the primary hemolysin response to sheep erythrocytes in the mouse has been shown to depend on the dosage, route, and time of administration of the endotoxin, as well as on the route employed for the injection of antigen. The normal production of antibody, following an intravenous or an intraperitoneal injection of red blood cells, is suppressed if the bacterial lipopolysaccharide is given before and by the same route as the antigen. The response to an intraperitoneal injection of sheep red cells is also inhibited if preceded by an intravenous injection of endotoxin. By contrast, hemolysin formation to intravenous antigen is enhanced considerably by a previous intraperitoneal injection of endotoxin, and the response both to intravenous and to intraperitoneal injections of the antigen increases if the endotoxin is given by the same route either simultaneously or shortly after the foreign red cells. These findings are discussed in regard to the physiological action of bacterial endotoxins and the early events in antibody formation.

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