1. Additional and conclusive evidence has been reported to substantiate the hypothesis that the extraction of lipids from antipneumococal sera, even though causing a loss of agglutinating and precipitating properties, in no way laters the primary and fundamental capacity of antigen and antibody to unite. Immunologic agglutination and precipitation are therefore entirely secondary phenomena and are probably dependent upon the presence of certain phosphatides, free or combined as secondary components of the antibody-complex.

  2. With respect to the nature of the lipid essential to these secondary reactions, type I antipneumococcal sera from 10 animal species fall sharply into two groups. Horse-antiserum is the prototype of the first class, the essential lipid being lecithin. The type-example of the second group is the antiserum of the rabbit, in which cephalin is required. The sera of the first class fail to give complement-fixation with pneumococcal polysaccharide, while those of the second give positive reactions. The free pneumococcal capsular polysaccharide is antigenic in the animals of the first class, while no antibody-response is obtained in animals of the second.

  3. The possible rôle of the essential lipids has been discussed.

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