The first complement component, C1, was activated in normal human sera by addition of aggregated IgG, immune complexes, and lipopolysaccharide, after which the concentrations of the C1 subcomponents, C1r and C1s, were measured by single radial immunodiffusion. A progressive reduction in C1r antigen content was observed as greater amounts of activator were added to normal human sera. Three lines of evidence proved that the apparent disappearance of C1r was due to interaction of C1r̅ with C1̄ Inactivator (C1̄ In). First, no loss of C1r antigen was detected in sera from patients with hereditary angioneurotic edema, which lack functional C1̄ Inactivator. Second, reconstitution of these patients' sera with physiologic concentrations of purified C1̄ In resulted in the disappearance of C1r antigen. Third, purified C1̄ In greatly reduced the intensity of the precipitin line formed between purified C1r̅ and anti-C1r̅.
In parallel experiments, certain antisera to C1s were found to show a comparable disappearance of C1s antigenic content in activated human sera. This antigenic loss was similarly shown to be due to the interaction of C1s̅ with C1̄ In. However, it should be noted that the detection of changes of C1s̅ antigenicity was dependent on the specificity of the C1s antisera employed, since antisera with antigenic specificities apparently directed to determinants on the C1s molecule distant from the region of interaction with C1̄ In did not show changes with activation.
Ultracentrifugal studies demonstrated that C1r̅ and C1s̅ formed soluble complexes with C1̄ In in activated serum, thus precluding the possibility that C1r̅ and C1s̅ escape immunochemical detection in activated serum by being present in large aggregated complexes. Also ruled out, in the case of C1s̅, is the possibility that the binding of C1̄ In leads to a major conformational rearrangement of C1s̅ with resulting changes in exposed antigens of the molecule. It is most likely that the loss in C1r̅ and C1s̅ antigenic content with activation of C1 in serum occurs because of simple physical masking of the relevant antigens of C1r̅ and C1s̅ by the relatively large C1̄ In molecule.