The equilibrium between three strains of influenza virus and their homologous antibodies is determined by two independent methods, with an accuracy of better than ±10%.

Counts of virus particles, of hemagglutinating and infective units are presented for the same preparations, and two techniques are developed for estimating the number of antibody molecules within sera (accuracy about ±30%).

From these data the number of antigenic sites per virus particle is calculated, and shown to be of the order of 2000 for each of the viruses tested. The equilibrium constants of the antigen antibody reaction fell within the range of 9 by 1010 and 5 by 1011 (in centimeter-gram-second (CGS) units), and were significantly different for each pair of reactants. These estimates carry over-all errors between ±25 and ±50%.

The parameters of virus-antibody interaction are also estimated by two special in vivo techniques. The number of antigenic sites per virus particle is the same as obtained by in vitro measurements, but the equilibrium constants are consistently lower. This both indicates some heterogeneity within the antibody population and points to the preferential involvement of the more firmly binding antibody molecules in the process of neutralization.

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