Immunoprecipitation (IP) is a technique for capturing specific proteins from a complex solution via antibody-antigen affinity. In an IP, the goal is to pull out a specific protein. For a co-IP, instead of identifying individual proteins, the goal of the technique is to identify protein complexes. The phrase “pulling down” protein is commonly used to explain the process, but this idea is somewhat dated now that magnetic nanoparticles have begun to replace traditional centrifuge-based methods. The protein capture efficiency can be measured by IP input into SDS page and western blot analysis.