Immunoglobulin (the academic name for antibody, and the words are often interchangeable) is a heterotetramer spherical molecule consisting of two pairs of heavy and light chains linked by disulfide bonds. In viviparous mammals, the heavy chain part of immunoglobulin can be divided into five subtypes according to the sequence: IgM (μ), IgD (δ), IgG (γ), IgA (α), IgE (ε), etc. In different species, there are sometimes more "subsubtypes", such as human IgG has four subsubtypes: IgG1 (γ1), IgG2 (γ2), IgG3 (γ3) and IgG4 (γ4), the mouse IgG has IgG1 (γ1), IgG2a (γ2a), IgG2b (γ2b), IgG3 (γ3) and other subtypes. The light chain can be divided into two subtypes: κ and λ. In addition to subtypes distinguished by sequence, antibodies can also be divided into two blocks, the antigen-binding region and the crystallizable region, according to the structure and function of the protein.

In general, the primary antibodies are mostly IgG antibodies, and the multi-strain IgG secondary antibodies can usually recognize the heavy and light chains of all subtypes, so the secondary antibodies can be recognized by the host IgG antibodies. Alternatively, secondary antibodies of a specific subtype can be used depending on the subtype of the primary antibody. For example, when the primary antibody is a mouse IgG2a antibody, anti-mouse IgG or anti-mouse IgG2a secondary antibody may be used.