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Studies of Antipneumococcal Serum

IV. Maximally Reactive Proportions of Antigen and Antiserum in Precipitation and Complement-Fixation

From the Division of Laboratories and Research, New York State Department of Health, Albany

Abstract

These experiments with sixty-six lots of antipneumococcal rabbit-sera have shown that the relative proportion of homologous type-specific carbohydrate and antiserum required for maximal fixation of complement in tests incubated for twenty-four hours at 3–6 C, designated as _3–6, was in all but one instance lower than the proportion of antigen: serum needed for maximal absorption, that is, for the most complete removal by precipitation of the complement-fixing and precipitative activity from the serum.

When antigen and antiserum were mixed in the proportions indicated as necessary for maximal absorption, the amount of complement fixed was similar to or significantly less than that observed when the lower antigen: serum proportion, _3–6, was used. Thus, these antisera appeared to have a greater combining capacity with antigen than was indicated by the antigen:serum ratio _3–6. Why this increased combination of antigen and antibody was not accompanied by a corresponding increase in the amount of complement fixed during twenty-four hours' incubation at 3–6 C remains a matter for conjecture.

Whereas, in precipitative tests, the intermediate zone appeared as a region in which the supernates exhibited activity neither with added antigen nor with added antiserum, in complement-fixation tests of the same supernates, the presence of both excess antigen and residual antibody was usually demonstrable. The extent of the intermediate zone in which appreciable complement-fixing activity with both added antigen and antiserum could be detected varied considerably for different antisera. It was narrowest for a number of individual bleedings, unheated previous to absorption; broadest for certain lots of processed sera concentrated by chemical methods. Irregularities were noted in the behavior of some seven antigen-preparations.

For ten of the sixty-six antisera tested, _3–6 fell in the region of slight antibodyexcess; for forty-three, in the intermediate zone toward or at the beginning of the zone of antibody-excess; for twelve, in the intermediate zone toward the region of antigen-excess; and for one serum, definitely in the zone of antigenexcess, as denoted by complement-fixation. In respect to the relation of _3–6 to the intermediate zone, the behavior of concentrated sera resembled that of the majority of unconcentrated sera. In most instances, therefore, tests of the complement-fixing activity of supernates from mixtures of antigen and antiserum in the proportion represented by _3–6 indicated the presence of a definite excess of antibody and a slight excess of antigen, whereas precipitative tests on the same supernates were consistently negative.