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Celiac disease built from mechanism through monitoring: modified gluten peptides bind HLA-DQ2/DQ8, drive T-cell inflammation, and trigger lymphocyte-mediated villous atrophy. The modern adult presents like IBS, iron deficiency, or an extraintestinal finding, so the threshold to test is broad. Diagnosis is serology-anchored and biopsy-confirmed while the patient still eats gluten, treated by a strict lifelong gluten-free diet with structured monitoring.
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Welcome to Board Pearls. This is episode one of two of the Celiac and Sprue Spectrum chapter, in the Stomach and Small Bowel Disorders module. This episode is celiac from mechanism through monitoring, and every fact in the chapter falls out of the mechanism, so start there.
Gluten peptides resist digestion because of their proline-rich structure, so they reach the lamina propria intact through a leaky barrier. There an enzyme, tissue transglutaminase, chemically modifies them, and that small change turns a poorly binding peptide into one that binds tightly to the celiac HLA molecules, DQ2 and DQ8. Antigen-presenting cells then show the modified peptide to gluten-specific T cells, which drive an inflammatory response, and B cells nearby make antibodies against both gluten and the modifying enzyme itself, which is why the antibody against tissue transglutaminase is the serologic marker. Meanwhile, stressed lining cells release a signal that expands the intraepithelial lymphocytes into killer cells that destroy the lining cells, and that destruction is the villous atrophy. So the whole disease is one chain: gluten, modification, HLA presentation, T-cell inflammation, antibody production, and lymphocyte-driven atrophy.
The HLA piece is the first non-negotiable element, and its clinical character comes from a mismatch. Nearly all celiac patients carry DQ2 or DQ8, but those alleles are also common in the general population while only a small fraction of carriers ever develop disease. So the test is excellent at ruling out, because a patient without either allele essentially can't have celiac, but poor at ruling in, because most carriers don't have it. That means HLA testing is used to exclude celiac, not to confirm it, and ordering it at the front of a standard workup is a mistake.
A few epidemiologic points worth carrying: the disease is bimodal, peaking in early childhood and again in middle age, somewhat more common in women, and it runs in families, so first-degree relatives are screened. And autoimmune comorbidity is the rule rather than the exception, type one diabetes, autoimmune thyroid disease, primary biliary cholangitis, Sjogren, and selective IgA deficiency, which matters for the serology, plus Down and Turner syndromes. The practical point is that any of these on the problem list lowers your threshold to screen, so iron-deficiency anemia in a young woman with type one diabetes should go to celiac serology before a broad test menu.
The clinical picture matters because the modern patient rarely looks like the textbook. Classic malabsorption with steatorrhea and weight loss is now the minority adult presentation. A large share present with symptoms that overlap heavily with diarrhea-predominant IBS, bloating, pain, alternating bowel habit, so the celiac vignette looks like the IBS vignette, and the teaching point is to draw celiac serology before accepting the IBS label. Iron-deficiency anemia is the next most common presentation, from lost duodenal absorptive surface plus a little occult blood loss, so the young woman with iron deficiency who fails oral iron or has no bleeding source is the textbook stem.
Extraintestinal findings are now the dominant presenting cluster, and any single one should trigger a celiac workup. Metabolic bone disease shows up as osteoporosis or a fracture in a young adult, from calcium and vitamin D malabsorption. Reproductive findings include infertility, recurrent miscarriage, and early menopause. Liver involvement appears as unexplained mild transaminitis that resolves on a gluten-free diet in most cases, and if it persists after gluten withdrawal, you pivot to autoimmune hepatitis or primary biliary cholangitis, which co-occur. Neurologic findings include peripheral neuropathy and gluten ataxia. And dental enamel defects, mouth ulcers, and a shrunken spleen with Howell-Jolly bodies on the smear round it out.
Three phenotypes to hold apart. Silent celiac is positive serology and biopsy-proven atrophy without symptoms, and it's treated anyway, because the bone disease and cancer risk accrue regardless of symptoms. Potential celiac is positive serology with normal villi and only increased lymphocytes, where treatment is debated and depends on trajectory. And seronegative celiac is biopsy-proven atrophy with negative antibodies in a patient with the compatible HLA who responds to gluten withdrawal, where the must-exclude mimic is olmesartan-induced enteropathy, a favored trap covered next chapter.
Dermatitis herpetiformis is the skin form of gluten sensitivity, treated as a celiac equivalent regardless of gut symptoms. It's an IgA antibody against a skin transglutaminase, distinct from the intestinal one, and the pathognomonic finding is granular IgA deposits in the dermal papillae on immunofluorescence of skin next to a lesion. The rash is intensely itchy blisters on the elbows, knees, buttocks, and scalp, usually excoriated by the time you see it. Every patient with it has celiac, every relative gets screened, and the treatment is a strict lifelong gluten-free diet, with dapsone added for the itch while the diet takes effect, remembering that dapsone needs G6PD testing first because of hemolysis risk and doesn't treat the gut disease. One coexisting condition to know: microscopic colitis occurs far more often in celiac patients, so ongoing watery diarrhea in a celiac with a healed duodenum needs a colonoscopy with random biopsies.
Now diagnosis, which is the core of the episode, and the principle is tested relentlessly: celiac is a serology-anchored, biopsy-confirmed diagnosis made while the patient is still eating gluten. The order matters, because withdrawing gluten before testing collapses both the antibodies and the histology, so a patient who's already gone gluten-free either does a gluten challenge before testing, the equivalent of a few slices of wheat bread daily for one to three months, or gets HLA testing to use its power to exclude. The first-line serology is the tissue transglutaminase IgA antibody, the best single test because it's highly sensitive and specific, and you draw a total IgA at the same time, because selective IgA deficiency is more common in celiac and will produce a falsely negative IgA-based test, in which case you pivot to an IgG-based test, the deamidated gliadin peptide IgG being the favored one. The endomysial IgA antibody targets the same thing and is highly specific but operator-dependent, used as confirmation when the transglutaminase antibody is very high.
The endoscopic confirmation needs a sampling protocol to own: at least four biopsies from the distal duodenum plus one or two from the bulb, and the reason for the bulb biopsies is that in a minority the atrophy is confined there, so skipping the bulb misses them. Endoscopic clues like scalloped folds and a mosaic pattern can hint at it, but the appearance alone is neither sensitive nor specific, so biopsy is mandatory. The histology is read on the Marsh classification, worth reciting in order: normal mucosa; then just increased intraepithelial lymphocytes, meaning more than twenty-five per hundred lining cells with normal architecture; then that plus crypt hyperplasia; then villous atrophy, which subdivides into partial, subtotal, and total flat mucosa. That's the histologic sequence.
The defining caveat is that the histology isn't specific on its own: not everything that flattens is celiac, and not everything that raises the lymphocyte count is celiac. Increased lymphocytes alone can come from H. pylori, NSAIDs, bacterial overgrowth, or post-infectious change, and villous atrophy can come from tropical sprue, autoimmune enteropathy, common variable immunodeficiency, the olmesartan and mycophenolate enteropathies, giardiasis, Whipple, lymphoma, or graft-versus-host disease. So the histology has to be matched to compatible serology and context. There is a no-biopsy pathway in adults, but it's narrow and not the default: a compatible clinical picture plus a transglutaminase antibody more than ten times the upper limit of normal plus a positive endomysial antibody on a second sample has a positive predictive value near certainty, so in that specific case an adult can be diagnosed without biopsy if they can't or won't proceed, but adult diagnosis still defaults to biopsy. HLA testing has a few valid uses, excluding disease in someone already gluten-free who won't do a challenge, adjudicating discordant serology and biopsy, and screening high-risk children, but it's not part of the initial algorithm.
Once diagnosed, you assess nutrition: a blood count, iron studies, B12, folate, vitamin D, calcium, magnesium, copper, and zinc, a baseline bone density scan, liver chemistries since transaminitis is common and usually resolves on the diet, and thyroid function given the comorbidity.
Now treatment and monitoring. A strict lifelong gluten-free diet is the entire treatment, and everything else supports and verifies it. You exclude wheat, barley, and rye and their relatives, while grains like rice, corn, quinoa, buckwheat despite the name, and others are allowed. Cross-contamination from shared production lines and surfaces is the dominant practical pitfall, which is why every new patient sees a dietitian experienced in celiac, and the threshold for a gluten-free label is under twenty parts per million, which supports mucosal healing in most patients. Oats are the favored caveat: their gluten-like protein is structurally different, so pure oats are tolerated by most patients, but with two catches, cross-contamination during production, and a small subset who react to the oat protein itself, so pure oats are generally acceptable, introduced cautiously with monitoring. And two under-recognized maintenance items: pneumococcal vaccination, because a functionally shrunken spleen raises the risk of encapsulated infections, and bone health, with that baseline density scan setting a surveillance schedule, repeated every few years, plus repletion of iron, calcium, vitamin D, and the other deficiencies, with oral iron absorption recovering as the duodenum heals.
Monitoring follows a schedule. The antibodies should normalize on a strict diet, falling by six months and normalizing by about a year in most patients, with very high starting titers taking longer, and you follow up every few months in the first year then less often, checking symptoms, adherence, and the nutritional labs. A persistently positive antibody after a year almost always means ongoing gluten exposure, so the next move is a dietitian re-evaluation for hidden gluten, not a refractory workup. But here's the most useful teaching point: a negative antibody doesn't guarantee the tissue has healed, because serology and histology can dissociate, so a patient can have clean antibodies and persistent atrophy. That's why the way to confirm healing at the tissue level is a repeat duodenal biopsy, recommended after about two years on the diet, sooner if symptoms or serology don't settle. Mucosal healing matters because it's tied to fewer long-term complications, particularly small-bowel cancer and lymphoma, which is the whole rationale for biopsy-based monitoring. So the vignette where symptoms have improved and serology is negative but the patient asks how to confirm healing is asking for a repeat biopsy.
Drug therapy is adjunctive at best. Several agents have been tried, a tight-junction stabilizer, a gluten-degrading enzyme for accidental ingestion, and others, and none is an accepted treatment, so the answer to "what treats celiac" remains a strict gluten-free diet with structured monitoring.
So pull it together. Celiac is an HLA-DQ2 or DQ8 gluten-driven enteropathy where modified gluten peptides activate T cells and a lymphocyte signal drives villous atrophy. The modern adult looks like IBS, or iron deficiency, or an extraintestinal finding, so the threshold to test is broad. The diagnosis is the transglutaminase IgA plus a total IgA while eating gluten, with the IgG-based test as the alternative in IgA deficiency, confirmed by at least four distal duodenal biopsies plus one or two from the bulb, read on the Marsh sequence. HLA is an exclusion tool, not a confirmation tool. Treatment is a strict lifelong gluten-free diet with a dietitian, nutritional repletion, a baseline bone scan, and pneumococcal vaccination, and monitoring uses serology and symptoms early with a repeat biopsy at about two years to confirm the healing that protects against the long-term complications.
Episode two picks up the patients who fail to respond or relapse after a year of strict diet. Most have a fixable explanation, but a true refractory population separates by the phenotype of their intraepithelial lymphocytes on flow cytometry: type one keeps a normal lymphocyte phenotype and behaves like aggressive conventional disease, while type two carries an abnormal clonal population that behaves as a low-grade T-cell lymphoma precursor, and that second group is where the enteropathy-associated T-cell lymphoma and ulcerative jejunitis arise.