What is reciprocal immunoprecipitation?

Reciprocal immunoprecipitation (R-IP) is an immunoprecipitation procedure often done as a form of confirmation for protein analysis, using an antibody against newly detected/identified proteins. When an experiment is performed and detects a complete new protein, or identifies a specific structure within the protein complex, R-IP will often be run against it. If it yields the original protein, (as determined using methods such as mass spectrometry) it is good evidence that the target protein was in fact the original complex, not of cross-reactivity or contamination. R-IP is not always necessary, but is a popular choice for demonstrating full rigor. The protocols of the procedure will be largely the same as any other IP, with the main difficulty in being selection of a suitable antibody.