Fab fragment antibodies are generated by papain digestion of whole IgG antibodies to remove the entire Fc portion, including the hinge region. These antibodies are monovalent, containing only a single antigen binding site. The molecular weight of Fab fragments is about 50 kDa.-Based on antigen-binding assay and/or ELISA, the antibody reacts with the heavy chain of rat IgM but not with rat IgG or the light chains of rat immunoglobulins. No antibody was detected against non-immunoglobulin serum proteins. The antibody may cross-react with IgM from other species.