Abstract
The usual dilution methods for determining the precipitin titer of antiserum only roughly approximate the relative antibody strength of a given serum and give no information with regard to the absolute quantity of antibody. Analysis of precipitates from the precipitinogen-precipitin reaction for their nitrogen content by the micro-Kjeldahl method has been used by Wu and his collaborators 1 and Heidelberger and Kendall. 2 Precise measurements of the antibody per unit volume of antiserum, and exact studies on the ratio of antigen to antibody in the precipitate have thus been made possible. This method requires at least 0.5 cc. of serum for each test and cannot be used when only small volumes of antiserum are available.
We have found that the following procedure gives satisfactory results for determining the quantity of precipitin in a small volume of rabbit anti-crystalline-egg-albumin serum. The method was suggested by figures presented by Heidelberger and Kendall. It depends on the determination of the amount of antigen necessary to precipitate the precipitins completely from a given volume of an antiserum. This determination of the mg. of antigen necessary completely to precipitate the antibodies from a sample of such serum is easily accomplished without analytical methods because the point of maximum precipitation proves to be that point where antigen as well as antibody is completely precipitated and neither antigen nor antibody remains in the supernatant fluid. We have called this optimum ratio of antigen to antibody the neutralization point. Furthermore, at this optimum point antigen and antibody combine in a ratio of about 1;13. The description of the method will clarify these points.
Method. The stock solution of crystalline egg albumin was diluted with saline to provide known concentrations of the antigen expressed in mg. of nitrogen per cc.
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