Immunoprecipitation (IP) vs. co-immunoprecipitation (co-IP)
The topic of co-immunoprecipitation (co-IP) is best preceded by an overview of immunoprecipitation (IP) to help frame an understanding of the principles involved. The description of IP methodology here is brief.
Immunoprecipitation is one of the most widely used methods for antigen detection and purification. The principle of an IP is very straightforward: an antibody (monoclonal or polyclonal) against a specific target protein forms an immune complex with that target in a sample, such as a cell lysate. The immune complex is then captured, or precipitated, on a beaded support to which an antibody-binding protein is immobilized (such as Protein A or G), and any proteins not precipitated on the beads are washed away. Finally, the antigen (and antibody, if it is not covalently attached to the beads and/or when using denaturing buffers) is eluted from the support and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), often followed by western blot detection to verify the identity of the antigen.
Schematic summary of a standard immunoprecipitation assay.
Watch this video to learn more about immunoprecipitation
Co-immunoprecipitation is an extension of IP that is based on the potential of IP reactions to capture and purify the primary target (i.e., the antigen) as well as other macromolecules that are bound to the target by native interactions in the sample solution. Therefore, whether or not an experiment is called an IP or co-IP depends on whether the focus of the experiment is the primary target (antigen) or secondary targets (interacting proteins).
Schematic summary of a standard co-immunoprecipitation assay.