Is Irritable Bowel Syndrome an Immune Problem?

Irritable bowel syndrome (IBS) is common, often triggered by stress or the flu. Evidence supports the idea that antihistamines might help people with abdominal pain.

If you often get cramps and diarrhea or constipation after meals, you’re hardly alone. Irritable bowel syndrome (IBS) is common, often triggered by stress or the flu. About 10 percent of us end up with ongoing intestinal distress after overcoming a challenge like food poisoning or a stomach virus.

If you see doctors for help, they’ll evaluate you for several intestinal problems that can have serious consequences. You may be checked for inflammatory bowel disease (IBD), a diagnosis that includes Crohn's disease and colitis, both of which involve immune disorders. If wheat products trigger your symptoms, you’ll take tests for celiac disease, an autoimmune reaction to gluten. Do you have a food allergy? People with IBS typically don’t turn out to have food allergies in a blood test.

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Evidence suggests that IBS may be a different kind of immune issue. Mast cells are immune cells especially abundant in the skin, airways, and intestines. They release histamine, among other chemicals, to fight infections.

Back in 2006, researchers discovered that in IBS patients some intestinal mast cells were acting as if the patients had an infection even when there was no pathogen present. The cells were also unusually close to nerve cells, triggering them to fire too much and potentially cause pain.

Why might this happen?

When you’re fighting an infection, your immune system may mistake proteins in foods as enemies. In some people with IBS, that mistake may linger long after the infection passes. To test this idea, researchers infected mice with harmful gut bacteria and fed them proteins from egg whites. When the infection was over, the team gave them the egg white protein again. The mice showed measurable stomach muscle contractions. The control group — mice that didn’t get the egg white protein during their infection — didn’t have this reaction post-infection.

In the first set of mice, the egg white protein triggered mast cells to release their defensive arsenals, chemicals that include histamine, the usual problem in allergic reactions. The unnecessary defensive response continued for four weeks, the length of the study.

This wasn’t exactly an allergy because the immune response stayed in the gut, rather than reaching the blood, where it could cause symptoms in other parts of the body. The mice didn’t have an egg white allergy.

A second set of experiments involved humans. The team tested 12 IBS sufferers for allergies to cow’s milk, gluten, wheat, and soy, and got negative results. But all of these volunteers had a local reaction to at least one of the four foods when it was injected into the rectum. Volunteers without IBS didn’t have any symptoms, by contrast.

Some people get the abdominal pain in response to stress rather than after an infection. The same team is now testing whether stress can trigger the unnecessary mast-cell reaction in mice.

What you can do

Mast cells release histamine, as noted earlier. You could talk to doctor about trying over-the-counter antihistamines. In a small clinical trial with 55 IBS patients over 12 weeks, an antihistamine cut IBS pain more than a placebo. However, the antihistamine in that study is not available in the United States. A U.S. study testing cetirizine (the chemical in Allegra, Zyrtec, and Claritin), as well as famotidine (the active ingredient in Pepcid) is underway. Why two? There are different kinds of histamine receptors: Ceterizine is a H-1 blocker and famotidine is a H-2 blocker.

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