Pepsin is a major proteolytic enzyme produced by the stomach. Pepsin digestion of IgG yields an F(ab') 2 fragment that includes the two antigen-binding (Fab) sites of the IgG molecule [5, 114, 196, 197] . Intact immunoglobulin, F(ab') 2 and other antibody formats are being exploited in development of antibody therapeutics [198] .