Section 01 · Molecular Mechanism
How Gliadin Breaks the Intestinal Barrier
Celiac disease is not a food intolerance. It is a chronic autoimmune condition in which dietary exposure to specific cereal proteins — collectively termed gluten — triggers an adaptive immune response that progressively destroys the absorptive surface of the small intestine.
Gluten is a composite of two protein families: gliadins (alcohol-soluble) and glutenins (insoluble), found in wheat, barley (hordein), and rye (secalin). The pathological driver is primarily the gliadin fraction — specifically alpha-, gamma-, and omega-gliadin peptides that resist complete digestion by luminal and brush-border proteases, leaving 33-mer and other immunogenic fragments intact in the intestinal lumen.
Step 2: Gliadin crosses the compromised epithelial barrier. In the lamina propria, the enzyme tissue transglutaminase 2 (tTG2) deamidates glutamine residues in gliadin peptides, converting them to glutamate. This increases the peptides' negative charge and dramatically enhances their binding affinity to HLA-DQ2 and HLA-DQ8 molecules.
Step 3: Antigen-presenting cells (dendritic cells, macrophages) display deamidated gliadin on HLA-DQ2/DQ8 to CD4+ T cells in the lamina propria. T cells differentiate toward a Th1 phenotype, secreting IFN-γ and TNF-α. The resulting cytokine environment activates CD8+ cytotoxic T cells and intraepithelial lymphocytes (IELs) that directly kill enterocytes.
Step 4: B cells produce anti-tTG IgA and anti-endomysial antibodies. Chronic inflammation drives crypt hyperplasia (crypts elongate to replace lost cells) and progressive villous atrophy (villi shorten and flatten), reducing absorptive surface area by up to 90% in severe cases.
Zonulin and the Leaky Gut Model
Gastroenterologist Alessio Fasano and colleagues identified zonulin — now understood to be the haptoglobin 2 precursor protein — as the body's master regulator of tight-junction permeability. Fasano's discovery that gliadin is one of only two known physiological triggers for zonulin release (the other being intestinal bacterial colonization) established celiac disease as the primary scientific model for intestinal hyperpermeability research.
When zonulin binds its receptor on epithelial cells, it activates a phospholipase C / protein kinase C signaling cascade that phosphorylates occludin and claudin proteins, physically opening the tight-junction gate. In celiac disease this is not a temporary physiological response — it becomes a sustained, pathological loop: gliadin enters, triggers zonulin, zonulin permits more gliadin entry, which triggers more immune activation.
Genetic Architecture: HLA-DQ2 and HLA-DQ8
The genetic predisposition to celiac disease is dominated by the HLA class II region on chromosome 6p21. Approximately 95% of celiac patients carry the HLA-DQ2 heterodimer, encoded by alleles HLA-DQA1*05 and HLA-DQB1*02. The remaining 4–5% predominantly carry HLA-DQ8 (DQA1*03 / DQB1*03:02).
These HLA molecules present deamidated gliadin peptides with unusually high affinity, explaining why immune activation is so consistent in carriers exposed to gluten. However, HLA-DQ2/DQ8 is necessary but far from sufficient: approximately 30–40% of the general population carries these alleles, but only 2–5% develop celiac disease. Additional genetic loci (IL21, IL2/IL21, TAGAP, and over 40 others identified in GWAS studies) and environmental factors — early microbiome colonization, infant feeding patterns, timing of gluten introduction, gastrointestinal infections — modulate conversion from genetic risk to clinical disease.
Section 02 · Histopathology
The Marsh Classification: Reading the Biopsy
The Marsh classification, modified by Oberhuber and later by Corazza, grades duodenal biopsy findings on a spectrum from normal mucosa to complete villous atrophy. Correct interpretation requires adequate sampling — guidelines recommend a minimum of four biopsy fragments from the second and third portions of the duodenum, with at least one from the duodenal bulb.
| MARSH GRADE | IEL COUNT (/100 epithelial) | CRYPTS | VILLI | CLINICAL CORRELATION |
|---|---|---|---|---|
| Marsh 0 | < 25 | Normal | Normal | Normal; not celiac |
| Marsh 1 | ≥ 25 | Normal | Normal | Increased IELs; seen in NCGS, H. pylori, NSAID use, latent celiac |
| Marsh 2 | ≥ 25 | Hyperplastic | Normal | Crypt elongation; uncommon presentation |
| Marsh 3a | ≥ 25 | Hyperplastic | Mild atrophy (villi/crypt > 1:1) | Partial villous atrophy; classic symptomatic celiac |
| Marsh 3b | ≥ 25 | Hyperplastic | Subtotal atrophy | Marked malabsorption; significant nutritional deficiencies |
| Marsh 3c | ≥ 25 | Hyperplastic | Total atrophy; flat mucosa | Severe disease; high complication risk; refractory disease marker |
A Marsh 3c flat mucosa means the villi have been completely effaced. The absorptive surface of the small intestine — which, if unrolled, spans approximately 250 square meters of effective area due to villi and microvilli — is reduced to a smooth, functionally near-useless tube. Malabsorption of iron, calcium, folate, fat-soluble vitamins, and zinc follows directly.
Section 03 · Serological & Histological Testing
Diagnosis: Serology, Biopsy, and the NCGS Distinction
Celiac disease diagnosis requires a systematic protocol. Serological tests are highly sensitive, but the gold standard remains duodenal biopsy — and both require the patient to be actively consuming gluten at the time of testing.
| Test | Sensitivity | Specificity | Notes | Strength |
|---|---|---|---|---|
| Anti-tTG IgA | 95–98% | 94–97% | First-line; requires normal total IgA. Titer correlates with histological severity. | HIGH |
| Anti-endomysial antibody (EMA) | 86–100% | 97–100% | Highly specific; operator-dependent immunofluorescence assay. Confirmatory role. | HIGH |
| Anti-deamidated gliadin peptide (DGP) IgA/IgG | 80–95% | 86–96% | Best test in IgA-deficient patients (use IgG version). Useful in young children <2yr. | HIGH |
| Total serum IgA | — | — | Mandatory first-step; IgA deficiency (prevalence ~1:400) causes false-negative tTG/EMA. | SCREEN |
| HLA-DQ2/DQ8 genotyping | ~99% NPV | Low PPV | Excellent rule-out value; not diagnostic. Useful when patient already on GFD. | EXCLUSION |
| Duodenal biopsy (≥4 samples) | Gold standard | Gold standard | Required for definitive diagnosis in adults. Marsh 2+ with positive serology is diagnostic. | GOLD STD |
Celiac vs. NCGS vs. Wheat Allergy
Celiac disease is autoimmune: positive serology (anti-tTG IgA, EMA), Marsh ≥2 on biopsy, HLA-DQ2/DQ8 carrier. Symptoms persist for years; long-term complications develop without GFD.
Non-celiac gluten sensitivity (NCGS) presents with gluten-related symptoms (bloating, fatigue, brain fog, abdominal pain) but negative celiac serology and normal duodenal biopsy. Diagnosis requires ruling out celiac and wheat allergy, then documenting symptomatic improvement on GFD and recurrence on rechallenge. The mechanism may involve innate immune activation via toll-like receptor 2 (wheat amylase trypsin inhibitors are emerging as a key driver) rather than adaptive immunity. There are no validated biomarkers for NCGS.
Wheat allergy is IgE-mediated: skin-prick test or specific IgE to wheat antigens is positive; symptoms are typically immediate (within 30–60 minutes) and can involve systemic anaphylaxis. Serological celiac markers are negative. Unlike celiac, wheat allergy may be outgrown, and barley and rye are usually tolerated.
Refractory celiac disease (RCD) is defined as persistent or recurrent malabsorptive symptoms with Marsh 3 histology after 12 months of strict GFD, having excluded inadvertent gluten exposure. RCD Type I has a normal IEL phenotype; RCD Type II features aberrant clonal IELs that lack normal surface markers — this carries up to 50% risk of progression to enteropathy-associated T-cell lymphoma (EATL).
Section 04 · Systemic Disease
Beyond the Gut: Extraintestinal Manifestations
Celiac disease is systemic. The intestinal lesion is often what gets diagnosed, but the presenting complaint may originate far from the duodenum. This is why celiac is both underdiagnosed in gastroenterology and chronically missed in other specialties.
Section 05 · Microbiome, GFD & Emerging Therapies
Microbiome Dysbiosis, Diet Adherence, and the Treatment Pipeline
The intestinal microbiome in active celiac disease is consistently altered. Studies using 16S rRNA sequencing find depleted Lactobacillus and Bifidobacterium populations alongside expansion of potentially pathogenic taxa including Bacteroides fragilis and Clostridium species. Whether dysbiosis is a driver of disease or a consequence of villous atrophy-induced substrate changes is unresolved, but the altered microbial community appears to persist on GFD, suggesting independent mechanisms are at play.
The only validated treatment for celiac disease remains the strict gluten-free diet. The FDA defines gluten-free as containing fewer than 20 parts per million (ppm) of gluten — the threshold below which controlled studies show no measurable mucosal injury in most patients. Hidden gluten is pervasive: shared cooking equipment, malt-based flavorings, soy sauce (wheat-fermented), oats processed in wheat facilities, and certain medications all represent contamination vectors. Cross-contact at restaurants — shared fryers, pasta water, shared surfaces — accounts for a significant proportion of ongoing mucosal damage in adherent patients.
Monitoring on Gluten-Free Diet
Anti-tTG IgA is the primary monitoring marker. Titers typically normalize within 6–12 months of strict GFD. Normalization of serology does not mean mucosal healing is complete — repeat biopsy studies show that full histological recovery (return to Marsh 0–1) takes 2 or more years in adults, and is incomplete in a substantial minority even after years of dietary adherence. Children heal significantly faster. Annual follow-up should include dietary review, tTG IgA, full blood count, ferritin, B12, folate, vitamin D, and bone density at baseline.
Emerging Pharmacological Treatments
Larazotide acetate is a tight-junction regulator that blocks zonulin-mediated permeability. Phase 2 trials demonstrated reduced gastrointestinal symptoms on a gluten challenge versus placebo. Phase 3 data are anticipated as an adjunct to GFD, not a replacement.
ALV003 (now IMGX003 / KumaMax) is an oral glutenase enzyme combination that degrades gluten in the stomach before it reaches the duodenum. Trials showed reduction in biopsy-determined mucosal injury during controlled gluten challenge. Intended as a mitigation tool for cross-contamination events, not as a dietary liberalization agent.
Nexvax2 was a gliadin-specific peptide immunotherapy designed to induce immune tolerance through regulatory T-cell expansion. Phase 2 trials were halted in 2019 due to failure to meet primary endpoints. The approach validated that HLA-DQ2-presented gliadin peptides can be targeted therapeutically, though the specific formulation did not achieve tolerance induction at the doses tested. Research continues in this class.
GutCode Protocol
The Celiac Workup Sequence: What to Ask Your Gastroenterologist
Before scheduling a biopsy or going gluten-free, ensure you have the full diagnostic picture. Initiating GFD before testing invalidates serology and biopsy results.
Order total IgA + anti-tTG IgA simultaneously. If IgA-deficient, order DGP IgG and consider anti-tTG IgG. Do not start GFD yet.
Positive serology (or strong clinical suspicion with equivocal serology) → upper endoscopy with ≥4 duodenal biopsies including bulb. Remain on gluten until procedure.
CBC, iron panel, ferritin, B12, folate, vitamin D 25-OH, zinc, bone density (DEXA). Identifies nutritional deficits for supplementation priority.
Work with a GFD-specialist dietitian. Repeat tTG IgA at 6 and 12 months. Expect normalization in 12 months; consider repeat biopsy at 2 years for mucosal confirmation.
Consider Lactobacillus/Bifidobacterium probiotic supplementation during recovery phase. Prioritize diverse, whole-food GFD to support microbial rebalancing.
First-degree relatives have 10% lifetime risk. Screen asymptomatic relatives with tTG IgA; consider HLA typing to identify zero-risk individuals who need no further monitoring.