Which immunoglobulin is usually on mucosal surfaces (secretory) and often low in nephropathies and immunodeficiency syndromes?

• A. IgA
• B. IgM
• C. IgG
• D. IgD

Answer: (A)

IgA is the antibody that predominates at mucosal surfaces and in secretions, existing as secretory IgA in places like the gut, airways, tears, saliva, and breast milk. It is produced as a dimer by plasma cells in mucosa-associated lymphoid tissue, joined by a J chain, and transported across epithelial cells to the lumen via the secretory (poly-Ig) receptor. The secretory component that accompanies IgA protects it from proteolysis, allowing it to function effectively in the harsh mucosal environment. Its main role is to neutralize pathogens and toxins at entry points, preventing adherence and invasion of organisms.

Because mucosal defense relies so heavily on IgA, conditions where IgA is low—such as selective IgA deficiency—lead to reduced mucosal immunity and higher susceptibility to infections at these sites. Some nephropathies and immunodeficiency syndromes are associated with low IgA levels, which is why IgA is the immunoglobulin described. The other antibodies aren’t primarily secreted at mucosal surfaces: IgM is mainly a serum pentamer involved in early responses, IgG is the principal circulating antibody for systemic immunity, and IgD mainly serves as a B cell receptor.