Protein immunoprecipitation can be a useful preparative step for immunoblotting.  For very rare proteins, the protein of interest can be purified and concentrated by standard immunoprecipitation techniques before immunoblotting.  In addition, protein-protein interactions can be tested with an immunoprecipitating antibody that is specific for one protein of a complex and an immunoblotting antibody that is specific for a second member of the complex.

Protocol: Immunoblotting: Preparing Immunoprecipitated Proteins Protocol