Immunoprecipitation (IP) is the technique of precipitating an antigen out of solution using an antibody specific to that antigen. This process can be used to enrich a given protein to some degree of purity. Co-immunoprecipitation (also known as a 'pull-down') can identify protein complexes present in cell extracts: by IPing one protein believed to be in a complex, additional members of the complex can also be identified.