The precipitin indices for a number of monovalent and polyvalent antipneumococcus sera were determined under known conditions, and found to vary as did the number of protective units. The ratio precipitin index/protective units in monovalent sera was found to lie between 2.8 and 4.8 for Type I and to be about ten times greater for Type III. Lower values were found in polyvalent horses and when mixing heterologous monovalent sera with each other. The influence of the duration of treatment upon the quotient was studied. Several refined and concentrated preparations showed a relative increase in precipitin activity.
The study of the precipitating reaction between antipneumococcus sera and the soluble specific substance derived from the three fixed types of Diplococcus pneumoniae promises a quantitative method for measuring the antibodies present in antipneumococcus sera. The relative simplicity in the chemical composition of one of the constituents, namely the soluble specific carbohydrate, paves the way for making this immunological phenomenon better understood.There is little doubt that the mutual compensation of electrical charges between two reacting colloids causes the phenomenon of precipitation (1, 2). So far colloidal chemistry offers an explanation for the mechanism of the reaction but not for the cause of its specificity toward that part of the colloidal serum constituents commonly known as antibodies. Avery, Heidelberger, and Goebel (3) demonstrated that the type specificity of the antigen is accounted for by distinct differences in chemical composition between the soluble specific substance of the three fixed types. The carbohydrates with their variability of configuration are apt to cause great physiological specificity, e.g. in the enzymatic hydrolysis of saccharides.On the other hand, the knowledge of the chemical nature or even of the concentration of the active specific principle of the other participant, the antibody, is so obscure that a differentiation of types from this angle seems futile at the present time.
The ratio precipitin/protective antibody is given for several fresh antipneumococcus horse sera (Type II). The application of the precipitin test here dealt with and that of similar ones, based on the conception of a parallelism between precipitin and protective antibody, is limited to unrefined horse sera.
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