A component that induces antibody production is called antigen . An antigen is usually bigger than the respective binding site of the antibody. The antibody-binding domain of the antigen is called an antigen determinant . At an immunization are as many different antibodies produced as antigen determinants are present. An antibody population is thus always heterogeneous or, as it is called, polyclonal .
Every antibody has two identical binding sites for antigen determinants.
Normally are rabbits used for the production of antibodies with a certain specificity. Two to three weeks after immunization can some of their blood be extracted. After centrifuging the blood cells down is an antiserum gained that contains the specific antibodies. They can now be used for the qualitative or quantitative detection of the used antigen or for the detection of substances that are similar to the antigen. In such cases is it spoken of serological cross-reactions or serological relationships.