Determination of antibody concentration using a stopped-flow turbidimetric technique.

The aggregation of albumin and anti-albumin is studied during the first seconds of the reaction by measuring the rate of change of the optical density of the mixture. When this rate is plotted against the antigen concentration of a constant serum dilution, bell-shaped curves are obtained. The maxima of these rate curves parallel the maxima of precipitation curves. Thus, the maximum rate establishes the antigen concentration for maximum precipitate. On the basis of these experiments a method for relative quantitation of antibody in sera is proposed. The absolute concentration of precipitating antibody can be obtained by means of an additional one-tube precipitation experiment.