The demonstration of linear relationships between serum and complement and between antigen and complement in the reaction of the complement-fixation test for syphilis (1) made possible the quantitative determination of the activity of serum. The increments of complement required to give 50 per cent hemolysis were directly proportional to the increments of serum used when the doses of antigen were adjusted to give the maximum reaction. Similarly, the increments of complement were directly proportional to the increments of antigen, provided the amount of serum used was in sufficient excess to give the maximum reaction obtainable. A considerable excess of serum appears not to diminish the reaction with a given amount of antigen; but, beyond a certain point, excess antigen yields lower reactions with a given amount of serum. Accordingly, improper adjustment of antigen content may result in prozone effects.

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