complement fixation - أطلس مختبرات العرب

The complement fixation test detects antibody. (1) A test antiserum is titred in doubling dilutions and a fixed amount of antigen is added to each tube or well. If antibody is present in the test serum, immune complexes will form. (2) Complement is then added to the mixture. If complexes are present, they will fix complement and ‘consume’ it. (3) In the final step, indicator cells (red cells) together with a subagglutinating amount of antibody (erythrocyte antibody) are added to the mixture. If there is any complement remaining these cells will be lysed; if it was consumed by immune complexes in stage 2, there will be insufficient to lyse the red cells. A quantity of complement is used that is just enough to lyse the indicator cells if none is consumed by the complexes. The assay is often performed on plastic plates. By using constant amounts of antibody and titrations of antigen, the assay can be applied to testing for antigens. Appropriate controls are most important in this assay because some antibody preparations consume complement without the addition of antigen, for example if the antibody preparation is serum that already contains immune complexes. Some antigens can also have anti-complement activity. The controls should therefore include antibody alone and antigen alone to check that neither fix complement by themselves.