Immunoglobulin Class/Fc Specific Secondary Antibodies

Antibodies targeted against the Fc portion of the primary antibody are class-specific.
There are three advantages of using Fc-specific secondary antibodies:

Use a Fc-specific secondary antibody for western-blotting in combined IP/WB experiments, if the protein of interest runs at an apparent molecular weight of about 25 kDa. Thus the signal derived from the light chain of the first secondary antibody used for immunoprecipitation does not mask signals around 25 kDa in western blotting of immunoprecipated material (if you also want to avoid masking of signals at 50 kDa use the ReliaBlot kit)
The Fc-specific secondary antibody is more specific for the immunoglubulin class of the primary antibody, since it does not react with the light chain that can be part of other immunoglobulin class antibodies (like IgM, IgA or IgE)
For double labelling studies with primary antibodies of the same species, it is a good idea to use Fab fragments as secondary antibody in the first labelling step and Fc-specific antibodies in the second labelling step in order to minimize cross-staining.

Here you find a selection of Fc-specific secondary antibodies from Bethyl: