What are the most common types of secondary antibodies?

Anti-mouse IgG and anti-rabbit IgG are the most common types of secondary antibodies. This is because most primary antibodies are generated in mouse or rabbit hosts and the species used to produce secondary antibodies must always be different from the primary antibody host and target species. For example, when using a primary antibody that is derived from a mouse, the anti-mouse secondary antibody must be generated in a species other than mouse.