Can fecal sIgA be elevated while fecal calprotectin is normal?

Fecal secretory IgA (sIgA) and fecal calprotectin are biomarkers that reflect distinct pathways of the intestinal immune system. While calprotectin is a hallmark of acute, innate inflammation, sIgA represents the "first line" of adaptive mucosal defense. Evidence indicates these markers can be decoupled, with sIgA rising in response to environmental or microbial stimuli even in the absence of the overt tissue inflammation required to elevate calprotectin.

Clinical evidence of biomarker discordance

Research suggests that sIgA and calprotectin often provide non-overlapping information regarding gut health. Because they track different biological processes, one can be elevated while the other remains within reference ranges.

• Subclinical Immune Activation: In patients with spondyloarthritis (SpA) who do not have overt inflammatory bowel disease (IBD), studies have identified elevated sIgA levels despite normal fecal calprotectin. This suggests a localized humoral response to gut antigens without significant neutrophil recruitment.
• Celiac and Dietary Triggers: Individuals with celiac disease, particularly those in the early stages of recovery or with low-level gluten exposure, may exhibit elevated sIgA while calprotectin remains normal (<50–100 μg/g). This indicates the adaptive immune system is actively neutralizing antigens (immune exclusion) before a full-scale inflammatory cascade is triggered.
• Irritable Bowel Syndrome (IBS): In IBS-D, patients frequently show increased sIgA-coated bacteria and higher total fecal sIgA, reflecting a response to dysbiosis, while calprotectin typically remains low, distinguishing functional disorders from inflammatory ones.

Mechanistic explanations

The divergence between these markers is rooted in the different cell types and signaling pathways involved in their production.

• sIgA and Immune Exclusion: sIgA is produced by plasma cells in the lamina propria after B cells are activated in the gut-associated lymphoid tissue (GALT). Antigens are sampled by M cells and presented to B cells, which then secrete dimeric IgA. This IgA is transported across the epithelium by the polymeric immunoglobulin receptor (pIgR). Its primary role is "immune exclusion"—binding to bacteria and toxins to prevent them from reaching the epithelial surface—a process that does not inherently require inflammatory cell recruitment.
• Calprotectin and Neutrophil Influx: Fecal calprotectin is a protein complex (S100A8/A9) found predominantly in neutrophils. It only appears in the stool in significant quantities when there is active migration of neutrophils across the intestinal mucosa, usually due to epithelial damage or potent pro-inflammatory cytokine signaling (e.g., TNF-α, IL-8).
• Thresholds of Activation: The gut can mount a robust sIgA response to "low-threat" antigens (like commensal bacteria or food proteins) via Toll-like receptor (TLR) signaling without reaching the threshold of "danger" required to trigger the massive neutrophil infiltration seen in IBD or infections.

Clinical implications

For practitioners and patients, an isolated elevation in sIgA suggests that the mucosal barrier is actively managing an increased antigenic load.

• Identifying Triggers: Elevated sIgA with normal calprotectin often points toward non-erosive triggers such as small intestinal bacterial overgrowth (SIBO), food sensitivities, or early-stage dysbiosis.
• Age-Related Considerations: In older adults (e.g., age 70+), the mucosal immune system may undergo immunosenescence, making sIgA a critical marker for assessing how well the gut barrier is adapting to age-related changes in the microbiota.
• Predictive Value: While not a replacement for calprotectin in diagnosing IBD, sIgA serves as a sensitive indicator of "functional" immune activity, highlighting a state of physiological stress on the gut lining before structural damage occurs.

Bottom line

It is biologically plausible and clinically observed that fecal sIgA can be elevated while fecal calprotectin remains normal. This pattern typically indicates an active adaptive immune response to luminal antigens (such as dysbiotic bacteria or dietary proteins) that has not yet—or may not ever—progress to the acute neutrophil-driven inflammation marked by calprotectin.