One of the five classes of immunoglobulins produced by humans (the others being IgE , IgD , IgG and IgM ).

IgA is found circulating and secreted on all defended body surfaces, as the primary defense against invaders. Secretory ( sIgA ) is found in large amounts in breast milk, saliva and gastrointestinal secretions. IgA may also be an important and effective antibody in sites other than mucosal tissues, such as the central nervous system. IgA inhibits the binding of micro-organisms to mucosal surfaces, thus preventing entry. sIgA deficiency is associated with increased gastrointestinal tract permeability and increased manifestations of delayed patterns of food allergy .
IgA has a dimeric structure and represents about 10-15% of the body's total immunoglobulin.