In an autoimmune disease, the immune system attacks the body's own tissue and causes damage that doctors can usually find with blood tests or biopsies. IBS doesn't work that way. It is diagnosed from a pattern of symptoms, standard tests show no structural damage, and it is classed as a disorder of gut–brain interaction.[1][4] The confusion is understandable, though, because the immune system does seem to play a part in some people with IBS.
What kind of condition is IBS?
IBS is a disorder of gut–brain interaction (DGBI). The term covers conditions in which the gut and brain communicate abnormally, producing symptoms through a mix of changes in gut movement, an oversensitive gut, altered immune function in the gut lining, altered gut bacteria and differences in how the brain processes gut signals.[1] US guidelines describe IBS as a chronic, often debilitating and highly prevalent DGBI.[2] In most people who see a doctor about it, symptoms come and go over years rather than clearing up in weeks.[3] See Can IBS be cured? for how it changes over time.
Why is IBS called a functional disorder?
For decades IBS was called a "functional" gastrointestinal disorder, meaning the symptoms are real but no structural or biochemical abnormality can be found to explain them.[4] The 2016 Rome IV criteria, which define IBS, moved toward the name "disorders of gut–brain interaction," partly to cut down on imprecise and sometimes stigmatizing terms.[1] UK guidelines now use the new name as well.[3] "Functional" never meant imaginary. It describes how the gut works, not what it looks like.
Is IBS inflammatory?
Not in the way inflammatory bowel disease (IBD) is. Crohn's disease, for example, is a chronic inflammatory disease that can lead to progressive bowel damage,[11] and stool tests for inflammation, such as fecal calprotectin, help doctors tell IBD apart from IBS.[2] But research has found subtle immune changes in the gut lining of some people with IBS:
- More immune cells. A meta-analysis of 22 studies, comparing 706 people with IBS with 401 controls, found more mast cells and T cells in colon biopsies from people with IBS, in both constipation- and diarrhea-predominant IBS. The changes varied by part of the colon, and they can't yet be used for diagnosis.[5] Extra mast cells are the most consistent finding under the microscope.[3]
- A leakier gut lining. Increased intestinal permeability has been linked with gut sensitivity and symptom severity in post-infection IBS and IBS with diarrhea, although findings in IBS with constipation are inconsistent.[3]
- Local reactions to food. In mice, a gut infection led the immune system to make IgE antibodies against a food, limited to the intestine; eating that food later activated mast cells and increased gut pain. When researchers injected food antigens such as gluten, wheat, soy and milk into the rectal lining of people with IBS, it caused local swelling and mast cell activation.[6] This is an immune reaction to food, not an attack on the body's own tissue.
UK guidelines describe this as low-grade mucosal inflammation and link it to past gut infections, stress, changes in gut bacteria and a weakened gut barrier.[3] These are research findings that help explain symptoms in some people. They aren't an established cause of IBS, and there is no biomarker that diagnoses it.[3]
How can an infection lead to IBS?
A gut infection is the best-documented trigger. In a meta-analysis of 45 studies covering 21,421 people, 10.1% had IBS 12 months after infectious enteritis, a risk 4.2 times that of people without such an infection. Women, people given antibiotics during the illness, and people with anxiety or depression were at higher risk.[7]
Some researchers have proposed an autoimmune-like mechanism for post-infection IBS. Antibodies against CdtB, a toxin made by some food-poisoning bacteria, may cross-react with vinculin, a protein in the gut wall. In a study of 2,375 people with IBS with diarrhea, these antibodies were higher than in people with IBD, celiac disease or no gut disease.[8] An Australian study, however, couldn't confirm that they separate IBS from other gut diseases, and found no difference in anti-vinculin levels between people with IBS and healthy controls.[9] For now this is an unproven idea, not evidence that IBS is autoimmune.
How is IBS different from celiac disease and Crohn's disease?
| IBS | Celiac disease | Crohn's disease | |
|---|---|---|---|
| What goes wrong | Disordered gut–brain communication and an oversensitive gut[1] | A permanent immune-mediated response to gluten in wheat, barley and rye[10] | Chronic inflammation linked to a dysregulated immune system in genetically susceptible people[11] |
| Damage to the bowel | None found on standard tests[4] | Injury to the small bowel lining[10] | Can cause progressive bowel damage[11] |
| Tests | No diagnostic biomarker; diagnosed from symptoms plus simple tests[3] | Specific blood antibodies, such as tissue transglutaminase, usually confirmed by biopsy[10] | Colonoscopy and biopsy, plus blood tests and imaging[11] |
| Main treatment | Diet, gut–brain therapies and medicines for symptoms[3] | A strict, lifelong gluten-free diet[10] | Medicines aimed at healing the inflammation[11] |
Because symptoms can overlap, guidelines recommend a celiac blood test for people with IBS symptoms (UK guidance says everyone at first presentation, US guidance those with diarrhea) and a fecal calprotectin test to help rule out IBD in people with diarrhea.[3][2] Having one condition doesn't rule out the other: IBS-type symptoms are common in people with IBD in remission and in people with celiac disease on a gluten-free diet.[14][15] Compare them in IBS vs celiac disease and IBS vs IBD.
When to see a doctor
IBS isn't known to turn into celiac disease or IBD, but in a large UK study a small number of people diagnosed with IBS were later found to have one of them, with the excess highest in the six months after the IBS diagnosis.[13] So see a doctor if your symptoms change, or if you have any of these features, which aren't typical of IBS:[12][2]
- blood in your stool, or black stools
- unintentional weight loss or anemia
- fever, or diarrhea or pain that wakes you at night
- a family history of IBD, celiac disease or bowel cancer
- symptoms that start after about age 50
The full list is in IBS red flags.