© 2007 Springer Science+Business Media, LLC. All rights reserved. In many species, including humans, IgA is produced in quantities that far exceed the combined synthesis of all other immunoglobulin (Ig) isotypes (Conley and Delacrox, 1987; Mestecky et al., 2005). It is estimated that in humans, <66 mg/kg body weight (i.e., 4-5 g in a 70-kg adult) is produced daily, in contrast with ∼25 mg/kg/day for IgG and much less for other isotypes. IgA displays diverse metabolic properties due to its heterogeneity of molecular forms (two subclasses, both of which can form monomers and polymers) and two distinct major sites of production (bone marrow and mucosal tissues) (Mestecky et al., 2005).