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

nent of complement (C'la) by human antibodies and human cells has been studied by the use of the C'la fixation and transfer test (C'la 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'la. Donath-Landsteiner antibody has the same re-quirements for C'la fixation. A single example of a warm agglutinin antibody which appears to re-quire one molecule of antibody for the fixation of C'la was found. Antibodies of the Rh system do not fix signifi-cant amounts of C'la in the absence of anti-anti-body when antiserum of a single Rh specificity was used. However, when three antisera at dif-ferent specificity are present, C'la 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-IgGHu antiserum and a one-to-one relationship between the concentration of anti-serum in the reaction and the amount of C'la fixed could be established. The effect of temperature, ionic strength, pa-painization of the red cells, and repeated washing of the red cell-antibody aggregates on the amount of C'la fixed was investigated. Conditions of maxi-Received for publication 28 December 1967 and in re-vised form 15 July 1968. mal C'la fixation were established for each class of antibodies. Globulins present in normal isologous or autolo-gous serum are absorbed in small amounts to nor-mal red cells in a manner analogous to warm ag-glutinin antibody. Their presence is detectable by the C'la fixation and transfer test only with anti-globulin antiserum. Within certain limits, the C'la fixation and transfer test provides a quantitative measure of the reaction of human red cells and antibodies to antigens on their surface