There is a thermoprecipitinogenic substance in extracts of the spleen of rats, guinea pigs and rabbits infected with T. equiperdum. It does not appear to be within the body of the trypanosome itself.

Antibodies to this heat-resistant precipitable substance were found in the serum of infected animals.

The antibody strength seems to be relatively less in the serum of rats than in the other animals but the power of extracts from the spleen of infected rats appeared to be equivalent to the power of similar extracts of the other animals.

The antibody titer of the serum of rabbits was greater than in the case of the other two species investigated. This was shown not only by the reaction with the extracts of spleens of the same species, but also by the reaction with extracts of the spleens of similarly infected animals of other species.

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