Extracts of Phase I H. pertussis suspensions containing toxin and agglutinin-absorbing antigen (agglutinogen) were prepared by sonic disruption of the cells. These were found to immunize mice actively against intracerebral infection with virulent pertussis bacilli and to cause the formation of agglutinin and antitoxin when injected into rabbits. When the extracts were absorbed with pertussis immune serum containing no antitoxin, they lost their agglutinin-combining properties but toxicity and ability to combine with antitoxin were not diminished. However, in the absence of agglutinin-absorbing antigen, the toxin no longer was measurably antigenic in rabbits, and failed to protect mice from intracerebral infection with pertussis. These experiments indicate that the toxin in the sonic extracts probably exists as a molecule separate from the agglutinin-absorbing or major protective component and by itself has little antigenic ability. In the presence of agglutinogen it becomes antigenic but its contribution to prophylaxis against H. pertussis infection, if any, is very small.

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