The present paper deals with reactions between antiserums and the capsules of Torula histolytica (Cryptococcus hominis or Torulopsis neoformans) and of Sporotrichum schenckii. The reactions, although similar in principle to the capsular swelling seen in Quellung tests of bacteria, present a special interest because the microorganisms are larger in size and more complex in structure than bacteria.

That the reactions on the capsules were caused by immunologically specific antibodies was shown by the fact that the capsular reactivity of the serums was directly related to their capacities to agglutinate suspensions and to precipitate soluble antigens of Torula and of Sporotrichum, and by the fact that the capsular reactivity was removed from the serums by absorption with soluble antigens of the respective fungi.

The soluble antigens included partially purified products from broth cultures of Torula and of Sporotrichum, a partially purified product from Sporotrichum-infected mice, and a purified polysaccharide from a culture of Torula grown in a synthetic medium. The purified polysaccharide was highly reactive in precipitation tests with unabsorbed serum, and its ability to absorb the Torula capsular reactivity from the serums indicates that the major antigens on the surface of encapsulated Torula cells are polysaccharides, identical with or similar to the product used in the present study.

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