Antigens used for complement fixation (CF) antibody assays are intended to detect specific antibodies of class IgM/IgG in mixture. The test can not discriminate between the antibody classes. The complement fixation test is performed in two stages. First, serum and antigen are mixed in presence of known amount of complement. If the serum antibodies and antigen react, the complement is bound to antigen–antibody complexes and depleted from the mixture. In the second stage erythrocytes with bound antibodies are added to the reaction mixture, and if complement remains from the first stage, the erythrocytes will be lysed. The highest serum dilution that prevents haemolysis is proportional to the concentration of antigen specific antibodies in the serum sample.

Influenza CF antigens are intended for the detection of class IgG and IgM antibodies to Influenza virus type A/H1N1 (A/H3N2, B or A and B) via complement fixation assay. Serum antibodies to Influenza type A CF antigen (B CF antigen or A and B CF antigen) are indicative of previous contact with the virus or with the Influenza antigens. Kits contain partially purified mixtures of formaldehyde inactivated Influenza virus type A (B) in allantoic fluid of chicken embryo in glycerol solution.