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.