The immunoglobulin that represents past exposure to an antigen is IgG. It is the predominant antibody in the secondary response and is an impoant defense against bacteria and viruses. In the secondary response, a much larger amount of IgG antibody is produced than IgM, and the levels of IgG tend to persist for a much longer time than in the primary response. IgA , the main immunoglobulin found in secretions, prevents the attachment of bacteria and viruses to mucosal surfaces. It does not bind complement. IgD probably functions as an antigen receptor. It is found on the surface of many B lymphocytes and is found in small amounts in serum. IgE is the primary antibody in type I hypersensitivity. It mediates the anaphylactic response and also paicipates in host defenses against parasites (antibody-dependent cell-mediated cytotoxicity-ADCC). Ref: Brooks G.F., Carroll K.C., Butel J.S., Morse S.A., Mietzner T.A. (2013). Chapter 8. Immunology. In G.F. Brooks, K.C. Carroll, J.S. Butel, S.A. Morse, T.A. Mietzner (Eds), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.