A 6-year-old boy from Mumbai is brought to the pediatric clinic with recurrent pyogenic infections (Staphylococcus aureus skin abscesses, recurrent otitis media, and pneumonia). Immunoglobulin levels are measured: IgG 280 mg/dL (normal 700–1600), IgA 15 mg/dL (normal 70–400), IgM 35 mg/dL (normal 40–230), IgE normal. Flow cytometry shows normal B-cell and T-cell counts. A diagnosis of selective IgA deficiency is suspected. Which structural feature of IgA makes it particularly important for mucosal immunity, and why does its deficiency predispose to recurrent infections at mucosal surfaces?

Explanation

## IgA Structure and Mucosal Immunity ### Unique Structural Features of IgA **Key Point:** IgA exists in two forms: 1. **Monomeric IgA** — found in serum (15% of total Ig) 2. **Dimeric IgA (secretory IgA, sIgA)** — the predominant form at mucosal surfaces (80% of total Ig produced) ### Dimeric IgA Structure Secretory IgA (sIgA) is a **dimer** composed of: - **Two IgA monomers** (each with 2 heavy chains and 2 light chains) - **J chain (joining chain)** — a 15 kDa polypeptide that covalently links the two monomers via disulfide bonds - **Secretory component (SC)** — a 70 kDa glycoprotein derived from the polymeric immunoglobulin receptor (pIgR) **High-Yield:** The **secretory component is critical** because it: 1. **Protects IgA from proteolytic degradation** by mucosal proteases (pepsin, trypsin, elastase) 2. **Facilitates transcytosis** across epithelial cells (pIgR-mediated) 3. **Enhances binding** to mucosal pathogens and toxins 4. **Provides structural stability** in the harsh mucosal environment ### Functional Advantages at Mucosal Surfaces ```mermaid flowchart TD A[Dimeric IgA + Secretory Component]:::action --> B[Transcytosis via pIgR]:::action B --> C[Secretion into mucus]:::action C --> D[Binds pathogens & toxins]:::action D --> E[Prevents epithelial adherence]:::action E --> F[Immune exclusion]:::outcome A --> G[Resistant to proteolysis]:::action G --> H[Survives harsh pH & enzymes]:::action H --> F F --> I[Reduced mucosal infection]:::outcome ``` **Clinical Pearl:** sIgA does **NOT** activate complement efficiently (because it cannot bind C1q) and does **NOT** opsonize for phagocytosis as effectively as IgG. Instead, its role is **immune exclusion** — preventing pathogen adherence and translocation. ### Why IgA Deficiency Predisposes to Mucosal Infections **Mnemonic:** **"No sIgA = No barrier"** In selective IgA deficiency: - **Loss of immune exclusion** at mucosae (respiratory, GI, urinary, genital) - **Increased pathogen adherence** to epithelial cells - **Reduced mucosal clearance** of toxins and antigens - **Unopposed bacterial colonization** → recurrent otitis media, sinusitis, pneumonia, and GI infections - **Normal serum IgG and IgM** cannot fully compensate because they do not reach mucosal surfaces efficiently (IgG is transcytosed poorly; IgM is too large) ### Clinical Features of Selective IgA Deficiency | Feature | Finding | |---------|----------| | **Serum IgA** | <7 mg/dL (normal 70–400) | | **IgG & IgM** | Normal or elevated | | **B & T cells** | Normal counts | | **Infections** | Recurrent mucosal (respiratory, GI) | | **Autoimmunity** | Increased risk (celiac, SLE, RA) | | **Anaphylaxis risk** | If transfused with IgA-containing blood | **High-Yield:** This patient's recurrent **pyogenic infections** (S. aureus abscesses, otitis, pneumonia) at mucosal/skin surfaces is the classic presentation of IgA deficiency. [cite:Robbins 10e Ch 6; Kuby Immunology Ch 4; Harrison 21e Ch 308]