Introduction to IgA: In human, the content of IgA in serum is second only to IgG, accounting for 10-20% of the total immunoglobulins, and has a unique role in mediating immunity. IgA is a polyvalent antibody, which is abundant in mucosal secretions. As the first line of defense against infection, IgA is the main defense mechanism for mucosal infections. Secretory IgA functions to prevent foreign substances from entering the circulatory system, rather than to destroy antigens.