Complement fixation on cell surfaces by 19S and 7S antibodies.

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.