Secondary antibodies help in the detection and sorting of antigens by binding to the primary antibody. These secondary antibodies are specific for specific species of the primary antibodies.
Secondary antibodies have applications throughout several types of assays, such as immunohistochemistry, Western blot, flow cytometry, and ELISA. The selection of secondary antibodies entirely depends on the class of primary antibodies (such as IgM, IgG), the type of label and the source host. Most of the primary antibodies belong to class IgG and they are generated in a common set of species of the host such as chicken, rabbit, goat, mouse, etc. Therefore, anti-goat IgG, anti-chicken, anti-rabbit IgG, anti-mouse IgG, or polyclonal antibodies are usually used.
For an optimal selection of a secondary antibody, we need to select an antibody that identifies the host species which was used to generate the primary antibody of interest. For example, we choose the anti-mouse secondary antibody to find a monoclonal primary antibody. Therefore, we need proper knowledge of detection assay to identify optimal secondary antibody. i.e ELISA and Western blot tests can be done by using fluorescence reporter system, chemiluminescent and colorimetric system while flow cytometry and immunofluorescence are usually limited to fluorescent reporter labels. In all these techniques, a conjugated secondary antibody is needed.