IgA

Immunoglobulin A (IgA) is an antibody isotype produced by plasma cells and is the main class of antibody found in mucous membranes and secretions. In the blood, IgA is primarily present as a monomer, while in secretions it exists mainly as secretory IgA, a dimeric form associated with a J chain and the secretory component.

IgA is divided into two human subclasses, IgA1 and IgA2. IgA1 is the predominant form in serum, whereas the distribution of IgA1 and IgA2 varies in mucosal tissues. The hinge region of IgA1 is longer and more susceptible to proteolysis by bacterial enzymes, while IgA2 generally shows greater resistance.

The production of IgA occurs mainly in mucosa-associated lymphoid tissue (MALT), including the gut and airway-associated lymphoid tissue, with additional contributions from bone marrow. Secretory IgA is transported across mucosal epithelia by the polymeric immunoglobulin receptor (pIgR); during this process, a portion of the receptor becomes the secretory component that remains bound to IgA in secretions such as saliva, tears, breast milk, and intestinal fluid. This secretory molecule stabilizes IgA in harsh mucosal environments.

Functions of IgA center on immune exclusion and neutralization at mucosal surfaces. SIgA can bind pathogens and toxins, preventing adherence and entry into the epithelium, and can neutralize microorganisms without triggering inflammatory cascades. IgA interacts with Fc alpha receptors on certain immune cells to mediate noninflammatory responses and aid in clearance, while typically having limited activation of the classical complement pathway.

Clinical relevance includes selective IgA deficiency, the most common primary immunodeficiency, and IgA nephropathy, in which aberrant IgA-containing immune complexes can deposit in the kidneys.