Fixation of the first component of complement (C'la) by human antibodies

The fixation of the first component of complement (C'1a) by human antibodies and human cells has been studied by the use of the C'1a fixation and transfer test (C'1a FT test) of Borsos and Rapp.Cold agglutinin antibodies appear to require no more than one antibody molecule to fix one molecule of C'1a. Most warm agglutinin antibodies are IgG in immunoglobulin type and require at least two molecules of antibody to fix a molecule of C'1a. Donath-Landsteiner antibody has the same requirements for C'1a fixation. A single example of a warm agglutinin antibody which appears to require one molecule of antibody for the fixation of C'1a was found. Antibodies of the Rh system do not fix significant amounts of C'1a in the absence of anti-antibody when antiserum of a single Rh specificity was used. However, when three antisera at different specificity are present, C'1a may be fixed. Under these conditions cells from a patient with paroxysmal nocturnal hemoglobinuria may be lysed when fresh serum is added to provide the other components of complement. The presence of IgG antibodies could be detected by the use of anti-IgG(Hu) antiserum and a one-to-one relationship between the concentration of antiserum in the reaction and the amount of C'1a fixed could be established. The effect of temperature, ionic strength, papainization of the red cells, and repeated washing of the red cell-antibody aggregates on the amount of C'1a fixed was investigated. Conditions of maximal C'1a fixation were established for each class of antibodies. Globulins present in normal isologous or autologous serum are absorbed in small amounts to normal red cells in a manner analogous to warm agglutinin antibody. Their presence is detectable by the C'1a fixation and transfer test only with antiglobulin antiserum. Within certain limits, the C'1a fixation and transfer test provides a quantitative measure of the reaction of human red cells and antibodies to antigens on their surface.