Studies on T H E Antigenic Properties of Complement I. Demonstration of Agglutinating Antibodies against Guinea Pig Complement Fixed on Sensitized Sheep Erythrocytes* By

The knowledge that exists of the role of complement in immune reactions has been gained from investigations limited almost entirely to studies in vitro. Very little is known of the role of complement in immune reactions in vivo. I t has not been demonstrated that complement fixation occurs in animal tissues in vivo in some way analogous to the fixation of complement to antigen-antibody aggregates in vitro. That some part of complement can actually become fixed to immune aggregates is indicated by the work of Heidelberger and associates who demonstrated the uptake of organic nitrogen by immune precipitates exposed to complement (1). However, such determination of the actual fixation of complement is applicable only to highly purified immune precipitates. Further elucidation of the fixation of complement by more complex immune aggregates, such as sensitized erythrocytes, could result if specific agglufinins against fixed complement could be experimentally produced and then used in an agglutination test with sensitized erythrocytes exposed to complement. The conjugation of such agglutinating antibodies with fluorescein could then also be employed to attempt visual demonstration of complement on sensitized erythrocytes. If this attempt were successful, it would appear reasonable to expect that complement fixed to sensitized cells in animal tissues might also be detected with the use of fluorescein-conjugated antibodies against complement.