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IgA is the most abundant immunoglobulin in body fluids and the second most abundant immunoglobulin in plasma, found at a concentration of 0.4 to 2.2 mg/ml. It plays a very important role in the first specific defense against natural infection. Secretory IgA differs from serum IgA in that it contains two additional peptides: the secretory component and the J chain.

In normal adult serum, the approximate percentage composition of IgA with respect to is subclasses is IgA1:90% and IgA2:10%. In secretory IgA, the subclass proportions may approach 50:50. The clinical significance of the subclasses has yet to be determined.