Question
IgA
Answer
IgA is the second most abundant immunoglobulin class in serum but is the predominant immunoglobulin in external secretions — saliva, tears, colostrum/breast milk, and respiratory/gastrointestinal/genitourinary mucosal secretions — making it central to mucosal defence.
Forms:
- Serum IgA — predominantly monomeric, circulates in blood, comprising ~10–15% of total serum immunoglobulin.
- Secretory IgA (sIgA) — a dimer of two IgA monomers joined by a J (joining) chain, plus an additional secretory component (SC), a fragment of the poly-Ig receptor acquired during transport across mucosal epithelium, which protects the molecule from proteolytic degradation in secretions.
Production and transport: synthesized by plasma cells in the mucosal lamina propria; the dimeric IgA-J chain complex binds the polymeric immunoglobulin receptor (pIgR) on the basolateral surface of the epithelial cell, is transcytosed across the cell, and released at the apical surface with the secretory component still attached.
Function: acts mainly by immune exclusion — binding and agglutinating microorganisms/toxins at the mucosal surface, preventing their adherence to and invasion through the epithelium; does not efficiently activate complement or opsonize (unlike IgG), acting instead as a “first line” mucosal barrier.
Clinical significance: provides passive mucosal protection to breastfed infants via colostrum, particularly important in the first days after birth; selective IgA deficiency is the most common primary immunodeficiency, predisposing to recurrent sinopulmonary and gastrointestinal infections, and, in some individuals, increased risk of atopic/autoimmune disease.