Secondary antibody broadly refers to the antibody that specifically reacts and binds to primary antibody, and in immunology experiments, it is often necessary to select different secondary antibodies for the needs of the experiment. Usually, there may be several types of secondary antibodies available for a particular experiment, and the following aspects need to be considered in order to select the most suitable secondary antibody for the experiment:

1) The source of the primary antibody
2) the class subtype of the primary antibody
3) the coupling labeling of the secondary antibody
4) Purification method of antibody

1. Species source of primary antibody
If the primary antibody is of a species origin, the corresponding secondary antibody should also be an antibody against that species. For example, if the primary antibody is prepared in mouse, then the secondary antibody should be anti-mouse, and if the primary antibody is prepared in rabbit, then the secondary antibody should be anti-rabbit. As to what kind of animal the secondary antibody itself is prepared in has no obvious effect on the quality of the secondary antibody.

2. Class subtype of primary antibody
The secondary antibody needs to match the class or subtype of the primary antibody. This is usually the case for monoclonal antibodies. Polyclonal antibodies are mainly IgG class immunoglobulins, so the corresponding secondary antibody is an anti-IgG antibody.
The class and subtype of the monoclonal antibody is usually listed in the product listing. If your primary antibody is a mouse IgM, then the corresponding secondary antibody should be either an anti-mouse IgM, or an anti-mouse IgG antibody.
If the monoclonal primary antibody is a mouse IgG subtype (IgG1, IgG2a, IgG2b, IgG3), then almost all anti-mouse IgGs will bind to it, or you can choose a secondary antibody that is specific to that subtype, e.g., if your primary antibody is a mouse IgG1, then you can choose an anti-IgG1 secondary antibody, which is particularly suitable for double-labeling experiments. This antibody is particularly suitable for double labeling experiments.
If you don't know which class or subtype the primary antibody is, then anti-mouse IgG is a good choice, as this antibody recognizes most types of IgG immunoglobulins.

3. Coupling Labeling of Secondary Antibodies
Generally speaking, the main probes coupled to the secondary antibody are HRP, fluorescein, biotin and so on. The choice of which probe to use depends mainly on the specific experiment. in ELISA, Western Blot and immunohistochemistry, the most commonly used secondary antibody is enzyme-labeled secondary antibody (e.g., HRP); whereas, in immunofluorescence and flow cytometry, fluorescein-labeled secondary antibodies are usually used, e.g., fluorescein such as FITC, CY3, PE, and so on. If greater amplification of the detection signal is desired, the Biotin/Avidin detection system can be used.

4. Antibody purification methods
Affinity chromatography purified antibodies are generally more popular because these products are more specific and produce less non-specific bands and background. However, there are cases where the loss of IgG during purification should also be taken into account, as the high affinity IgG fraction is particularly important when containing antibodies with high affinity, especially when the target antigen is present in small amounts.

See

How to choose antibodies wisely?

Principles of Western Blot Internal Reference Antibody Selection