Pathology · patient case question
What is the principal protective function of secretory IgA in saliva?
Patient case: The mouth that will not stay wet
demo: 54-year-old female
chief: Burning, sticky mouth for the past eight months; food gets caught and she wakes at night to sip water.
history: Hashimoto thyroiditis. Her rheumatologist ordered an autoantibody panel after she reported gritty, burning eyes.
meds: Levothyroxine 75 mcg daily. Amitriptyline 25 mcg at night for chronic tension headache.
findings: Anti-SSA (Ro) strongly positive. Salivary pooling is absent; the mucosa is dry and the tongue is fissured with papillary atrophy. Cracking and erythema at both commissures. Two new cervical lesions since her last visit a year ago.
Show answer and explanation
Correct answer: C. It blocks bacterial adherence to oral surfaces by immune exclusion
- A. Direct bacterial lysis is the job of lysozyme, which hydrolyses peptidoglycan, and of the complement membrane attack complex. IgA does not lyse organisms.
- B. Secretory IgA is a poor complement activator and does not use the classical pathway; this restraint is deliberate, since it avoids inflammation at a surface constantly exposed to commensals.
- C. Correct. Secretory IgA works by immune exclusion: it agglutinates organisms and coats their adhesins so they cannot attach to mucosa or tooth surfaces, and are cleared by swallowing. Preventing adherence is the key step it interrupts.
- D. Opsonisation for phagocytosis is chiefly an IgG function through Fc gamma receptors on neutrophils and macrophages.
Note: The recall recorded the option pair as antibacterial versus antiviral. Secretory IgA is in fact both, neutralising viruses as well as blocking bacterial adherence, so the discriminating idea is the mechanism of immune exclusion rather than the target organism.