On the Mechanism of Complement Fixation

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Abstract
(1) If a series of dilutions of an antiserum are prepared, a dilution can be selected which when mixed with an appropriate quantity of the homologous antigen forms no precipitate but nevertheless binds complement. If complement is added to such a mixture of antigen and antiserum a precipitate appears after an interval of six to twenty-four hours. (2) By using an appropriate mixture of antigen and antiserum the amount of the precipitate can be increased, within certain limits, by increasing the amount of guinea-pig serum (complement) present in the mixture. (3) Similar results can be obtained if an euglobulin solution prepared from guinea-pig serum is substituted for the normal guinea-pig serum. (4) A suitable mixture of antigen and antiserum precipitates the euglobulin of guinea-pig serum in a manner which may be compared with the precipitation of euglobulin by carbon dioxide. (5) By keeping a, precipitating mixture of antigen, antiserum and complement at a temperature of 0° C. it is possible to demonstrate that the resulting precipitate contains the mid-piece fraction of the complement in an active state. The mid-piece fraction is not used up when it is bound by a mixture of serum with its homologous antiserum. (6) The precipitate which results from the interaction of antigen and antiserum fixes both fractions of the complement. The mid-piece is fixed much more readily than the end-piece but it is possible to demonstrate a stage when a considerable quantity of end-piece has been bound while a small quantity of mid-piece still remains free. (7) The results obtained in these experiments show that the fixation of the fractions of the complement by a mixture of antigen and antiserum is essentially similar to the fixation produced by suspensions of barium sulphate and similar complement fixing substances. These experiments confirm the recent work of Gengou on this subject. (8) The particles of a precipitate probably adsorb the euglobulin of the guinea-pig serum and this adsorption of euglobulin is an essential part of the mechanism of complement fixation.

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