IgA (Immunoglobulin A) represents only 10-15% of the total Ig molecules in serum but is the predominating Ig isotype in saliva, tears, breast milk and in the gastrointestinal and urogenital tract. In serum, the IgA molecule is commonly found as a monomer, but can also exist as dimers or larger polymeric forms. The IgA found in secretions (secretory IgA) is commonly a dimer with two IgA molecules joined by the J chain. Secretory IgA is the most important Ig isotype in mucous surfaces, the area of entry for most pathogens.