Crohn’s causes:
the inside story.

We don't know exactly what causes Crohn's, but through research we have a better understanding of what's involved. Let’s uncover what’s known.

On this page, you’ll find info about:

Immune System Icon

The power (and problem) of the immune system

It protects the body

The immune system is a network of cells, tissues, and organs that work together to protect the body. One way it does that is through inflammation.

The body can trigger an immune response

Normally, harmless bacteria in the gastrointestinal (GI) tract (many of which aid in digestion) are left alone by the immune system. But with Crohn’s, these bacteria are mistaken for harmful invaders—triggering the immune system to send cells to the GI tract and produce inflammation.

Inflammation contributes to Crohn’s symptoms

When there’s active inflammation in the (GI) tract, it may lead to symptoms and damage inside the body, such as:

  • Ulcers 
  • Thickening of the intestinal wall
  • Diarrhea
  • Abdominal pain

If there is chronic inflammation and symptoms, medication is a treatment option that doctors can consider.

Inflammation: the bigger picture

Since inflammation can travel throughout the body, what does that mean for someone with Crohn’s disease? It means that inflammation can be found anywhere along the GI tract...and beyond. 

Symptoms & Complications
Click/tap on the interactive tool below to learn more about how and where Crohn’s disease can impact you.


Since Crohn’s can affect the entire GI tract, that means it can also affect where it all begins: the mouth. Symptoms may include ulcers, which cause pain and discomfort.


Crohn’s disease can lead to ulcers in the esophagus. Symptoms may include heartburn and chest pain.


Crohn’s disease can lead to chronic inflammation and additional problems in the stomach. Symptoms may include stomach pain, nausea, and vomiting.

Small intestine

Crohn’s disease inflammation can thicken the wall of the intestines and cause bowel blockage, abscesses, ulcers, and fistulas.

Large intestine (Colon)

Crohn’s disease can lead to ulcers in the colon that can deepen or spread to surrounding tissue if inflammation levels increase. Symptoms may include abdominal pain, diarrhea, and fever.


Crohn’s disease can lead to ulcers in the rectum and perianal fistulas in the anus.


Crohn’s disease can lead to a stricture (narrowing) in the intestines, colon, rectum, or anus. Some symptoms of strictures may include abdominal pain, vomiting, and bloating. Talk to your doctor right away if you experience any of these symptoms.


Crohn’s disease can lead to a fistula, an abnormal passage between two organs (for example, the small intestine and the large intestine), or between an organ and the outside of the body. About 30% of people with Crohn’s may develop fistulas.

Depending on which organs it affects, a fistula may include symptoms like infection of the urinary tract, gas, and painful bumps on the skin.


Crohn’s disease can lead to an abscess (a collection of pus, which can develop in the abdomen, pelvis, or around the anal area).

Signs that there is a complication with an abscess may include severe abdominal pain, painful bowel movements, and discharge of pus from the anus.

Why does Crohn’s disease happen?

It’s a simple question—but a simple answer is harder to come by. That’s because the exact cause of Crohn’s disease is still unknown; however, there are three major factors that are thought to contribute to someone’s chance of experiencing Crohn’s.

Immune System Response:

In people with an inflammatory bowel disease like Crohn’s, the immune system overreacts to something in the GI tract, resulting in inflammation.

Genetics and Heredity:

Certain genes are connected to someone’s chances of developing the disease, but having the genes doesn’t mean that the disease will develop. 

Environmental Factors: 

Environmental factors can include smoking, antibiotics, nonsteroidal anti-inflammatory drugs, appendicitis, and diet. While the role these factors play is not fully known, they can serve as “triggers” to initiate a harmful immune response in the GI tract.

Next page:

With the causes of Crohn’s reviewed, let’s learn more about Crohn’s Disease Severity & Progression

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