On the Mechanism of True and of Non-Specific Complement Fixation Reactions

Abstract

The complement fixation test for tuberculosis gives a non-specific or falsely positive reaction if a balanced physiological saline, akin to Tyrode's solution, replaces the plain 0.9% NaCl used in the test. The balanced saline which gives the strongest non-specific reaction contains 0.888% NaCl, 0.009% CaCl₂, and 0.003% MgCl₂; both Ca and Mg salts are required, and it is only over a very limited range of concentration of these salts that the non-specific reaction occurs. Petroff's 1 whole bacillus antigen is more effective than any other examined in producing this non-specific reaction, while “fat-free ”tuberculo-antigens do not give this non-specific effect. In the absence of antigen, i. e., in the anticomplementary control tubes, balanced saline assists hemolysis, so that one-third of the normal unit of complement may produce complete hemolysis.

The mechanism of this falsely positive reaction is as follows:

1. The surface properties of the (Petroff) antigen particles are modified when incubated with complement and plain saline, presumably due to irreversible adsorption of protein from the diluted guinea pig serum used as complement; the isoelectric point of the washed antigen is changed from pH 3.2 to pH 3.7 by this treatment.

2. In the presence of balanced saline, the surface properties of the antigen undergo a further change, in that the particles agglutinate readily, and fix complement; i. e., the antigen now acts as does an antigen sensitized by immune serum. The tendency of the antigen to agglutinate is a reversible one, which is present while the antigen is in balanced saline, and absent if the antigen is transferred to plain saline.

Further details of the factors involved in this reaction will be given elsewhere. 2 The critical Na to Ca ratio which will produce this non-specific reaction is also approximately the ratio of these ions present in the blood and in those balanced physiological salines which are essential to many forms of cell life; it is also the Na to Ca ratio, which, due to antagonism of the Na and Ca ions, exerts zero effect upon the stability of oil-water emulsions (Clowes 3 ).