Do elevated fecal secretory IgA and calprotectin reflect different types of intestinal immune activity?

In a 57-year-old man, fecal calprotectin and secretory IgA provide different, non-equivalent information about intestinal immune activity. Calprotectin has established clinical utility as an inflammation marker; total fecal secretory IgA is more contextual and assay-dependent.

Clinical evidence

• Fecal calprotectin: Elevated concentrations reliably reflect intestinal neutrophil-associated inflammation and generally correlate with endoscopic and histologic inflammatory activity. It is useful for detecting or monitoring active mucosal inflammation, but it does not diagnose inflammatory bowel disease (IBD) or identify its cause. Many adult assays use <50 µg/g as an analytical reference range; 100–250 µg/g is often treated as intermediate and >250 µg/g as more suggestive of substantial inflammation.
• Fecal secretory IgA (sIgA): An elevated total concentration can plausibly accompany increased intestinal mucosal immune activity, but it is not a validated quantitative measure of the degree of “immune engagement.” There is no universal adult reference interval or clinically established high-sIgA threshold.

Mechanistic interpretation

• sIgA is produced by mucosal plasma cells, transported across enterocytes by the polymeric immunoglobulin receptor (pIgR), and released into the gut lumen with secretory component. It participates in host–microbiota interactions, including IgA targeting of intestinal bacteria.
• Calprotectin is released in association with neutrophil migration into the intestinal lumen; fecal levels therefore reflect neutrophil-rich mucosal inflammatory activity.

Practical interpretation

• Total fecal sIgA can be influenced by infection, inflammation, diarrhea or transit, IgA status, specimen handling, and assay methodology. Patterns of bacterial IgA coating may be more biologically informative than bulk sIgA concentration.
• Calprotectin can rise with infection, NSAID or PPI exposure, coeliac disease, diverticular disease, colorectal cancer, and other organic gastrointestinal conditions. Symptoms, medications, infection evaluation, and—when indicated—endoscopy with biopsy determine the cause.

Bottom line

• Elevated calprotectin is a well-supported marker of intestinal neutrophil-associated inflammation; elevated total fecal sIgA is a plausible but nonspecific signal of mucosal immune–microbiota activity, not proof of increased immune engagement.