The sensitivity and specificity low, but used in diagnostic screening,since gives an accurate method of measuring the amount of antigen or antibody present. Need to inactivate complement present in antiserum. The actual mechanics of this assay similar to hemagglutionation. The classical pathway of complement is activated by the binding of the C1q subunit of the C1 macromolecule to IgG or IgM which is either aggregated or complexed with the antigen. This results in a cascade of proteins.
Complement enhances the neutralization of virus by antibody.
This is acheived by increasing the size of the virus antibody complex and by producing incorporation of C3b into complexes which can then bind to receptors on phagocytic cells. Antibody and complement can directly lyse those viruses with lipid envelopes. Antibody and complement can kill virus infected cells in vitro. A variety of human cell lines infected with viruses which contain surface antigens are lysed by human serum if both antibody and functional complement are present. Advantage is the use of single tagged reagent for many antibodies.