IBD and Stool Secretory IgA Levels
Inflammatory Bowel Disease causes HIGH levels of IgG (not sIgA) in stool and intestinal mucosa, while paradoxically showing LOW or unchanged secretory IgA levels compared to the expected mucosal immune response.
Key Immunoglobulin Patterns in IBD
Elevated IgG Production
- Mucosal IgG is dramatically elevated in active IBD, with median levels of 512 μg/ml in active ulcerative colitis and 256 μg/ml in active Crohn's disease, compared to only 1.43 μg/ml in irritable bowel syndrome controls 1
- This represents a shift from the normal intestinal pattern where IgA predominates to an abnormal IgG-dominant response 1, 2
- IgG synthesis in IBD intestinal tissue increases tenfold compared to controls, particularly IgG directed against commensal bacterial proteins 1, 2
Secretory IgA Response
- IgA synthesis per million cells remains unchanged in IBD despite active inflammation 2
- IgA antibody titers against fecal bacteria are actually lower in ulcerative colitis than controls (p < 0.01), suggesting impaired secretory immune function 1
- Total mucosal IgA concentrations do not differ significantly between IBD patients and controls 1
Clinical Significance
The Immunoglobulin Shift
- The exaggerated IgG response (rather than the normal IgA response) represents a breakdown of tolerance to normal commensal gut flora 1
- This IgG is directed primarily against cytoplasmic bacterial proteins, indicating loss of normal mucosal barrier function 1
- The pattern is more pronounced in Crohn's disease than ulcerative colitis 1
Systemic Immunoglobulin Levels
- Serum IgG levels are frequently low in IBD patients (22.7% have low IgG, 23.4% have low IgG1) due to protein-losing enteropathy and chronic inflammation 3
- Risk factors for low serum immunoglobulins include increasing age, disease duration, hypoalbuminemia, and thiopurine use 3
- Low IgG/IgG1 levels are associated with worse outcomes including increased need for surgery in Crohn's disease 4
Diagnostic Implications
The question specifically asks about sIgA, which is NOT elevated in IBD. The characteristic finding is:
- High mucosal/fecal IgG (indicating pathologic immune activation) 1, 2
- Normal to low secretory IgA (indicating impaired normal mucosal immunity) 1, 2
- This pattern distinguishes IBD from normal intestinal immune responses where IgA predominates 1
Monitoring Inflammation
- Fecal calprotectin (not immunoglobulins) is the preferred biomarker, with levels <50 μg/g reassuring for ruling out active inflammation 5, 6
- Values between 50-250 μg/g require further evaluation as they may indicate low-grade inflammation 5
- Serial monitoring at 3-6 month intervals facilitates early recognition of disease flares 5, 6