The estimationin vitroof small amounts of diphtheria antitoxin by means of a haemagglutination technique

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Abstract
1. Treatment of washed sheep erythrocytes with dilute solutions of tannic acid rendered them capable of adsorbing diphtheria toxin from solution. Diphtheria toxoid was not adsorbed under similar conditions.2. Cells thus sensitized with diphtheria toxin were agglutinable by a proportion of children's sera and human cord sera; sera of rabbits, mice and horses, immunized with soluble or adsorbed diphtheria toxoid, also agglutinated sensitized cells, as did also the sera of children suffering from diphtheria and treated with antitoxic serum. Clear-cut results were usually obtained, but the end-point of titrations on horse sera was, in some cases, difficult to determine.3. Using intradermal toxin neutralization tests, a highly significant linear relationship was demonstrated between the haemagglutinating titres of children's sera for the sensitized red cells, and the antitoxin content of their sera. The antitoxin content corresponding to the smallest detectable amount of haemagglutinating antibody was 0·008 unit per ml. serum.4. The haemagglutinating titre was an accurate measure of the mean antitoxin level of a group of sera and a close measure of the antitoxin content of most specimens in individual tests.
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