Which immunoglobulin is primarily found in mucosal secretions and often has a secretory component?

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IgA is the immunoglobulin that dominates in mucosal secretions such as saliva, tears, mucus, and intestinal fluids. It is typically secreted as a dimer joined by a J chain and then transported across mucosal epithelium by the polymeric immunoglobulin receptor. During this transport, a portion of the receptor remains attached to the IgA as the secretory component, producing secretory IgA. This secretory component protects IgA from proteolytic enzymes in the harsh mucosal environment and helps stabilize it in secretions. Secretory IgA is specialized for immune exclusion: it neutralizes pathogens and toxins before they can attach to and penetrate mucosal surfaces, providing a first line of defense with minimal inflammatory damage. Other antibodies have roles elsewhere—IgG is primarily systemic, IgM is an early responder often as a pentamer, and IgE is involved in allergic responses and parasite defense—so IgA best fits being the antibody most associated with mucosal secretions and a secretory component.