- The complement fixation test is one of the most established serology methods. This relies on the formation of specific antibody-antigen complex that activate the complement system. Serum for antibody testing is mixed with known target antigen. Any antibodies present will react with antigens, make immune complex and use up any complement that has been deliberately added. Complement use is measured by adding red cells treated with red cell antibody. Residual complement, left over from binding specific antibody-antigen complex, completes the reaction that lyses the red cells