The levels of IgA and IgG in human external secretions are well below serum levels and the IgA is dominant in most secretions. Human IgA is heterogenous with respect to its molecular form and subclasses. Plasma or serum IgA is primarily represented by monomeric (m) IgA with two heavy and two light chains. A variable proportion of IgA in human external secretions is present in the form of mIgA and pIgA devoid of secretory component (SC). Most S-IgA in humans is derived from local synthesis and not from the circulation. A relatively high proportion of Ig-producing cells in the mucosal lymphoid tissue is committed to the IgA isotype. This fact-in conjunction with the presence of the specific pIgR expressed on mucosal epithelial cells-accounts for the high relative concentration of IgA in mucosal secretions. In most species, the concentration of IgG in mucosal secretions is approximately the same as or greater than that of IgM. The proportion of IgA to IgG in external secretions not only varies from site to site but also depends on the method and time of collection. © 2005 Elsevier Inc. All rights reserved.