Determination of avidity of anti-viral antibodies at 50% binding of antibody.

Binding studies with tobacco mosaic virus and specific IgG and Fab' fragments were done under conditions of 50% binding of antibody. When the log of the concentration of antigen sites required to bind 50% of the available antibody was plotted against the log of bound sites, a line was obtained which had a slope varying between 0 and 1. Instead of using the slope of this line as an index of avidity of the antiserum, it is suggested that the calculation K0 values over a range of reactant concentrations provides a better criterion for comparing the avidity of antiviral antibodies. The results of conventional binding tests were analyzed by means of Scatchard plots and were found to agree very closely with the data obtained at 50% binding of antibody.