Can IBS cause low-grade mucosal immune activation and visceral hypersensitivity even with normal fecal calprotectin?

Evidence-based clinical research confirms that in Irritable Bowel Syndrome (IBS), both low-grade mucosal immune activation and visceral hypersensitivity frequently persist even when fecal calprotectin levels are within the normal range. This occurs because the standard biomarkers for gut inflammation are designed to detect neutrophil-driven pathology, which differs significantly from the subtle immune dysregulation seen in IBS.

Clinical and diagnostic evidence

Fecal calprotectin is a highly sensitive marker for the neutrophil-mediated inflammation characteristic of Inflammatory Bowel Disease (IBD). However, studies demonstrate that in IBS, calprotectin levels typically remain below the diagnostic threshold (often <50–100 μg/g).

• Clinical data shows that normal calprotectin results do not exclude microscopic immune activity; for instance, significant increases in T-lymphocytes and mast cells are frequently observed in patients whose calprotectin levels suggest a "non-inflammatory" state.
• The lack of correlation between calprotectin and symptom severity indicates that this biomarker cannot reliably detect the low-grade histological changes that drive IBS symptoms.

Mechanistic explanations

The presence of pain and hypersensitivity despite "normal" lab work is explained by specific neuro-immune interactions:

• Cellular Shift: While IBD is neutrophil-dominant, the low-grade activation in IBS involves an increased density and degranulation of mast cells and T-lymphocytes (CD3+ IELs).
• Neuro-Immune Proximity: Mast cells in IBS patients are often located in closer proximity to enteric nerves. These cells release mediators such as tryptase, histamine, and prostaglandin E2, which directly sensitize nociceptors.
• Receptor Sensitization: This process upregulates receptors like TRPV1 and protease-activated receptors (PARs) on colonic afferent nerves, lowering the threshold for pain and causing normal gut sensations to be perceived as distress.

Clinical implications

For patients, particularly those in older demographics where comorbid conditions may complicate the clinical picture, these findings validate that gastrointestinal distress is not "imaginary" despite normal inflammatory markers. The absence of elevated calprotectin rules out overt colitis but does not address the underlying neuro-immune sensitivity that dictates symptom severity and quality of life.

Bottom line

Low-grade mucosal immune activation and visceral hypersensitivity are primary drivers of IBS that function through mast cell and lymphocyte pathways, meaning they are invisible to neutrophil-based tests like fecal calprotectin. Normal test results should not be used to dismiss the physiological basis of chronic abdominal pain or hypersensitivity.