1. Type-specific M extracts and group-specific C carbohydrate of group A streptococci inhibited bacteriostasis of these microorganisms in the presence of normal whole blood and of sera from patients convalescent from streptococcal infections. The inhibition was not specific with respect to streptococcal types and depended merely on the formation of precipitates in the system. The extracts had no antagonistic action in themselves.

2. Preformed precipitates derived from the interaction of an antigen and its homologous antibody or from finely divided coagulated particles of human plasma or ascitic fluid also interfered with the bacteriostasis. The supernatant fluid in which one of these precipitates was formed did not inhibit bacteriostatic activity; therefore, it seems that other possible products of the antigen-antibody reaction were not inhibitory. The relative size of the precipitate particles was a conditioning factor since small particles of one precipitate inhibited bacteriostasis, but large ones of the same precipitate failed to do so.

3. Stained films of blood cells treated with antigen-antibody mixtures which formed a precipitate revealed large cytoplasmic vacuoles containing precipitates in the polymorphonuclear leukocytes and monocytes; such engorged cells subsequently failed to phagocytize streptococci in homologous serum. Blood cells treated in the same manner, except that the antigen-antibody mixtures formed no precipitate, contained no vacuoles, and these cells were able to phagocytize the streptococci.

4. Leukocytes studied in the living state in the presence of colored precipitate and streptococci sensitized by convalescent human serum showed unselective phagocytosis of both precipitate and bacteria. The capacity of these leukocytes to ingest material however was limited.

5. As a result of non-selective saturation of their phagocytic capacity in the bacteriostatic systems containing both streptococci and precipitate, the limited number of leukocytes phagocytized only a fraction of the streptococci; consequently the remainder were able to multiply.

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