Immunoglobulins (Igs) play key roles in immune protection of mucosal surfaces that form an
immense and vulnerable interface with the environment. In humans, the principal Ig in most
but not all mucosal secretions is IgA. In some secretions, IgG and IgM contribute toward
immune defense. IgA in secretions comprises polymeric (chiefly dimeric) forms of IgA made
up of two monomers of IgA, each made up of two heavy or alpha chains and two light chains,
linked together via J or joining chain, and covalently associated with secretory component, a …

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 …