The Perfect Immunological Storm
Coeliac disease requires three converging elements: dietary gluten (specifically gliadin peptides from wheat, and homologous prolamins from barley and rye), the HLA-DQ2 or HLA-DQ8 genotype (present in nearly all coeliac patients, but also 30 to 40 percent of the general population), and environmental triggers that breach tolerance. The result is a T cell-mediated adaptive immune response that is unique among autoimmune diseases in having an identified exogenous trigger.
Gliadin's Unusual Resistance to Digestion
Gluten proteins are unusually rich in proline and glutamine residues, which resist cleavage by gastric, pancreatic, and brush border peptidases. The resulting large peptide fragments — particularly a 33-amino-acid fragment from alpha-gliadin — survive transit to the small intestinal lamina propria intact, retaining their immunogenic potential.
The tTG Amplifier
In the lamina propria, the enzyme tissue transglutaminase (tTG) deamidates specific glutamine residues on gliadin peptides, converting them to negatively charged glutamic acid. This deamidation dramatically increases the binding affinity of these peptides for the HLA-DQ2 or DQ8 groove on antigen-presenting cells. The modified peptides are now potent activators of CD4+ T helper cells, which mount a Th1-dominant immune response with production of IFN-gamma, TNF-alpha, and IL-21.
Simultaneously, tTG-gliadin complexes stimulate B cells to produce antibodies — anti-tTG (the basis of the serological screening test), anti-endomysial antibodies, and anti-deamidated gliadin peptide antibodies. The autoantibody against tTG itself is a hallmark that makes coeliac disease a true autoimmune condition with an identified autoantigen.
The Destructive Cascade
The CD4+ T cell response drives two parallel destructive pathways. Cytokines activate lamina propria fibroblasts and matrix metalloproteinases that remodel the extracellular matrix, causing crypt hyperplasia. Meanwhile, IL-15 — produced by stressed epithelial cells — activates intraepithelial lymphocytes (IELs) expressing NKG2D receptors, which directly kill enterocytes bearing the stress ligand MICA. This cytotoxic IEL pathway is partly independent of the adaptive immune response and is particularly important in refractory coeliac disease.
The net result is progressive villous atrophy: the finger-like villi that maximise absorptive surface area flatten, crypts elongate to compensate, and IELs infiltrate the epithelium — the histological triad captured by the Marsh-Oberhuber classification.
Permeability and Zonulin
Gliadin also activates the zonulin pathway through the CXCR3 receptor on enterocytes, triggering phosphorylation and disassembly of tight junction proteins (ZO-1, occludin). The resulting increase in paracellular permeability allows more gliadin peptides to reach the lamina propria, creating a feed-forward loop. This zonulin-mediated barrier disruption is one of the earlier events in disease pathogenesis and may precede full villous atrophy.
The entire cascade is reversible: strict gluten avoidance removes the antigenic stimulus, inflammation subsides, tight junctions reassemble, and villi regenerate — typically within 6 to 24 months, though some patients require longer.