The possible use of complement for the detection of cell surface antigens.

The modern trend in complement research concerns the functionally relevant interactions of complement components and the biologic activities of the complement system. The pri mary interest of students of complement today centers on complement as a humoral effector system which constitutes an integral part of the immunologically competent organism. There exists only a secondary interest at present in complement as a serologie reagent used in the laboratory for the detection of antigen-antibody complexes. Consequently, little work has been done so far on the use of complement for the detection of cellular antigens. The discussion of this subject will, therefore, have largely a theoretical character. The following presenta tion will include a brief description of the chemical nature of complement components and of the essential features of the complement reaction mechanism. On the basis of this informa tion, those properties of individual complement components and of individual reaction steps will be pointed out which may be suitable to serve as sensitive indicators of antigen-antibody complexes and, hence, of cell surface antigens. The complement system (most of the present discussion will deal with human complement) consists of eleven distinct serum proteins or nine complement components, which, in terms of protein, account for approximately 10% of the serum globulin fraction.3 Of the eleven human complement proteins, seven have thus far been obtained in a high degree of molecular homogeneity [C'lq (25), C'ls (8), C'2 (17, 33, 35), C'3 (30), C'4 (23), C'5 (30) and C'94 (7)]. The other four components