The mechanism of complement fixation on cell surfaces by whole antiserums, and by 19S and 7S fractions has been studied with a new comple-ment-fixation test. This test is based on the fixation and transfer of the activated first component of complement (C1a). We have concluded that a single molecule of 19S antibody in combination with antigen at the cell surface is sufficient to bind one molecule of C1a. For 7S antibodies at least two molecules in close proximity at the cell surface are required to fix one molecule of C1a.

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Abstract. Herpes simplex virus which had been sensitized with immunoglobulin M antibody was neutralized by serum deficient in the fifth and sixth components of complement (C) but not by serum deficient in the fourth component C (C4). The sequential addition of the functionally purified components of C showed that the activated first component of C (Ci) failed to neutralize sensitized virus. However, in the presence of an optimal concentration of C1, the addition of C4 resulted in neutralization Under certain conditions, antisera against herpes simplex virus (HSV) produce little or no neutralization unless complement (C) is present.1-4 Little is known, however, about the mechanism by which complement neutralizes virus. Recent studies on the mechanism of action of anti-y-globulin on virus sensitized with antiviral antibody suggested that attachment of the anti--y-globulin to the antiviral antibody on the sensitized virion resulted in neutralization by more extensive coverage of the surface of the virion than occurred with antiviral antibody alone.44 This pointed to the possibility that the mechanism of neutralization of sensitized virus by C also might be due to increased coverage of the virion surface by the attachment of C. Extensive studies on the mechanism of action of C on erythrocytes sensitized with anticell antibody showed that there are at least nine components of C and that these components act in a definite sequence.7 Furthermore, some of these components are capable of attaching to the anticell antibody and to the cell surface. Most of the components of C are now available in functionally pure form and this made it possible to study the individual 528