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 Fc5mu portion of the human IgM heavy chain but not with human IgG, IgA, or the light chains of human immunoglobulins. No antibody was detected against non-immunoglobulin serum proteins. The antibody may cross-react with IgM from other species.