A 28-year-old man from Delhi presents with recurrent sinopulmonary infections, chronic diarrhea, and failure to thrive since infancy. Serum immunoglobulin levels are: IgG 180 mg/dL (normal 700–1600), IgA <7 mg/dL (normal 70–400), IgM 45 mg/dL (normal 40–230), and IgE <2 IU/mL (normal <150). Flow cytometry shows normal B and T cell counts. A diagnosis of selective IgA deficiency is made. Which structural feature of IgA makes it the predominant immunoglobulin at mucosal surfaces despite its low serum concentration in this patient?

Explanation

## IgA Structure and Mucosal Immunity ### Structural Features of Secretory IgA (sIgA) **Key Point:** IgA exists in two main forms: monomeric IgA (serum) and dimeric secretory IgA (sIgA) at mucosal surfaces. The dimeric form is stabilized by the J chain (joining chain) and secretory component (SC), which are essential for mucosal immunological function. ### Structural Components of Secretory IgA | Component | Structure | Function | |---|---|---| | **IgA monomer** | 2 heavy chains (α) + 2 light chains | Basic antibody unit | | **J chain** | ~15 kDa polypeptide | Covalently links two IgA monomers; produced by plasma cells | | **Secretory component (SC)** | ~70 kDa glycoprotein | Derived from polymeric Ig receptor (pIgR); protects sIgA from proteolysis; enhances mucosal adherence | | **Dimeric sIgA** | Two IgA units + J chain + SC | Stable, resistant to proteases; optimal for mucosal defense | **High-Yield:** The J chain is synthesized by IgA-secreting plasma cells in mucosal lymphoid tissues and covalently links two IgA monomers to form dimeric IgA (dIgA). The secretory component is added post-secretion when dIgA binds to the polymeric immunoglobulin receptor (pIgR) on epithelial cells. ### Why sIgA Dominates at Mucosal Surfaces 1. **Dimeric structure:** Two IgA units provide increased valency and avidity for multivalent antigens. 2. **Secretory component:** Protects IgA from degradation by mucosal proteases (trypsin, pepsin, elastase). 3. **Mucosal targeting:** pIgR-mediated transcytosis delivers dIgA across epithelial cells to secretions (saliva, tears, breast milk, respiratory/GI secretions). 4. **Immune exclusion:** sIgA binds pathogens and toxins, preventing their adherence to epithelial cells. **Clinical Pearl:** In selective IgA deficiency, patients lack mucosal sIgA despite normal or near-normal IgM and IgG. This explains recurrent sinopulmonary and GI infections — the first line of mucosal defense is compromised. ```mermaid flowchart TD A[IgA-secreting plasma cells<br/>in mucosal lymphoid tissue]:::action --> B[Synthesis of IgA monomer<br/>+ J chain] B --> C[Dimeric IgA + J chain<br/>dIgA] C --> D[Binding to polymeric Ig receptor<br/>pIgR on epithelial cells]:::action D --> E[Transcytosis across epithelium]:::action E --> F[Cleavage of pIgR<br/>Secretory component remains attached]:::action F --> G[Secretory IgA sIgA<br/>in mucosal secretions]:::outcome G --> H[Immune exclusion<br/>Pathogen neutralization<br/>Protection from proteolysis]:::outcome ``` **Mnemonic:** **JCSA** — **J** chain, **C**omponent (secretory), **S**table, **A**ntigen exclusion. **Warning:** Do not confuse IgA monomers (serum) with dimeric sIgA (secretions). The J chain and secretory component are what make sIgA effective at mucosal surfaces — without them, IgA is rapidly degraded by proteases.