Evidence is presented that phenomena ascribed to the properdin system may be explained in terms of classical antibody (Ab) in combination with three of the components of complement (C'), i.e., C'1, C'4, and C'2 respectively. The results suggest that the complex of properdin (P) with zymosan (Z) represents a mixture of Z·Ab, Z·Ab·C'1, 4, Z·Ab-C'1, 4, 2, and Z·Ab·C'1, 4, 2 in a decayed state.

The purported preferential reactivity of Z with the third component of C' (C'3) is not supported by the present experiments in which Z was reacted with C' in both guinea pig and human sera. Approximately the same number of reactive units of C'1, 4 and of C'3 were inactivated by Z, as well as by D. pneumoniae and a washed specific precipitate of bovine albumin-anti-albumin.

The latter evidence that small amounts of antigen-antibody complex fix significant amounts of C'3 stands in contrast with the classical concept that C'3 is not fixed in ordinary complement fixation reaction. The observed reactivity of C'3 is explained on the basis of the present use of essentially undiluted serum as a source of C'3 and of 37° as the temperature for fixation.

Ancillary data indicate that purified properdin contains Ab. In the presence of a chelating agent and at 0° properdin agglutinated Z granules. Measurements of nitrogen (N) uptake by Z suggest that 0.25 µg. N were contributed by solutions stated to contain 1 unit of properdin.

The broad spectrum of reactivity or cross-reactivity of the Ab in normal serum is likely due to the wide distribution in nature of closely related polysaccharides. Further immunochemical studies are necessary to establish definitively the origin and mode of action in "natural resistance" of antibodies reactive with these polysaccharides.

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