Secretory immunity: Which immunoglobulin class is predominantly found in external secretions such as saliva, tears, colostrum, and mucosal fluids?

Introduction / Context:
Mucosal surfaces are key portals of pathogen entry. Secretory immunoglobulin A (sIgA) protects these surfaces by neutralizing pathogens and toxins without provoking inflammation.

Given Data / Assumptions:

• Focus is on external secretions: saliva, tears, milk/colostrum, respiratory and intestinal mucus.
• We consider the dominant Ig class present as secretory dimeric IgA.

Concept / Approach:
sIgA is produced by plasma cells in mucosa, dimerized via J chain, and transported through epithelial cells using the polymeric Ig receptor. The secretory component protects IgA from proteolysis in harsh mucosal environments.

Step-by-Step Solution:
Identify the immunoglobulin enriched in secretions → IgA. Note structure: dimer with J chain and secretory component. Recall function: neutralization at mucosal surfaces without complement-mediated inflammation. Select IgA among options.

Verification / Alternative check:
Clinical measurements (e.g., salivary IgA) and mucosal immunology literature confirm IgA predominance in secretions.

Why Other Options Are Wrong:
IgD is a B-cell receptor isotype; IgM predominates in primary serum responses; IgE mediates allergy and parasite defense; IgG dominates serum and tissues.

Common Pitfalls:
Confusing serum abundance (IgG) with secretory predominance (IgA).

Final Answer:
IgA.