Secretory IgA (sIgA) biology: The secretory component associated with dimeric IgA in mucosal secretions is derived from which source/mechanism?

Introduction / Context:
Secretory IgA is the dominant immunoglobulin in mucosal secretions such as saliva, tears, and intestinal fluid. Its resistance to proteolysis and effective mucosal transport depend on a specialized secretory component attached to dimeric IgA.

Given Data / Assumptions:

• Plasma cells in lamina propria produce dimeric IgA linked by J chain.
• Epithelial cells express a polymeric immunoglobulin receptor (pIgR).
• Secretory component originates from epithelial cells, not lymphocytes.

Concept / Approach:
Dimeric IgA binds to pIgR on the basolateral surface of epithelial cells. The complex undergoes transcytosis to the apical surface. There, a proteolytic cleavage releases IgA into the lumen with a residual portion of pIgR still attached—this remnant is the secretory component, which stabilizes IgA and protects it from proteases.

Step-by-Step Solution:
Identify producer of IgA: plasma cells create dimeric IgA with J chain. Recognize epithelial transport via pIgR. Note apical cleavage of pIgR yields the secretory component. Select the option describing pIgR cleavage during transcytosis.

Verification / Alternative check:
Immunohistochemical studies show pIgR expression on mucosal epithelia and colocalization with sIgA at the apical surface, confirming epithelial origin of the secretory component.

Why Other Options Are Wrong:

• Plasma cells / T cells / Dendritic cells: Do not produce secretory component; they produce IgA or regulate responses.
• M cells: Specialized for antigen sampling, not for adding secretory component.

Common Pitfalls:
Assuming all parts of sIgA are lymphocyte-derived; in fact, secretory component is epithelial in origin via pIgR processing.

Final Answer:
Formed by cleavage of an epithelial polymeric Ig receptor during transcytosis.