IgA
What it means
Dominant antibody at mucosal surfaces — a secretory dimer with a J chain and secretory component (saliva, tears, gut, respiratory tract, breast milk → neonatal mucosal protection). Selective IgA deficiency is the commonest primary immunodeficiency (often asymptomatic; risk of anaphylaxis to IgA-containing blood products).
Worked example
Anti-tissue-transglutaminase used to screen coeliac disease is an IgA antibody — check total IgA too, because IgA deficiency gives false negatives.
In the exam
The predominant antibody in breast milk and gut secretions exists as a dimer with a secretory component protecting it from luminal proteases.
What settles it
IgA = mucosal/secretory dimer; commonest primary immunodeficiency. IgE = mast-cell/allergy; IgM = pentamer.
Classically confused with
IgE
Source: StatPearls — Immunoglobulin