(IP) enables the purification of a protein (an antigen) from a mixture, taking advantage of the natural antibody/antigen affinity. This method not only allows the concentration of proteins of interest that would otherwise be difficult to detect in applications such as western blotting. IP can also help identify protein-protein, protein-DNA or protein-RNA interactions by immunoprecipitating their complexes. Basically, a protein mixture or cell lysate containing the protein of interest is incubated with a specific antibody solution. The antibodies will recognize and specifically bind to the corresponding antigens (proteins of interest). Antibodies are either previously immobilized on a solid-phase support (e.g. super-paramagnetic beads or non-magnetic agarose beads) or will be bound to protein A or G which has been immobilized on a solid-phase support. The solid support is sedimentable and thus indirectly precipitates the antibody/antigen complexes. Antigens can then be retrieved by dissociating the complexes.