A ‘Safe House’ for Bacteria
Such studies add weight to a theory that Dr. Parker and his colleagues developed back in 2007: That the appendix acts as a “safe house” for beneficial gut bacteria.
Think of the colon as a wide pipe, Dr. Parker said, that may become contaminated with a pathogen such as Salmonella. Diarrhea follows, and the pipe gets repeatedly flushed, wiping everything clean, including your friendly gut microbiome. Luckily, “you’ve got this little offshoot of that pipe,” where the flow can’t really get in “because it’s so constricted,” Dr. Parker said. The friendly gut microbes can survive inside the appendix and repopulate the colon once diarrhea is over. Dr. Parker and his colleagues found that the human appendix contains a thick layer of beneficial bacteria. “They were right where we predicted they would be,” he said.
This safe house hypothesis could explain why the gut microbiome may be different in people who no longer have an appendix. In one small study, people who’d had an appendectomy had a less diverse microbiome, with a lower abundance of beneficial strains such as Butyricicoccus and Barnesiella, than did those with intact appendixes.
The appendix likely has a second function, too, Dr. Smith said: It may serve as a training camp for the immune system. “When there is an invading pathogen in the gut, it helps the GI system to mount the immune response,” she said. The human appendix is rich in special cells known as M cells. These act as scouts, detecting and capturing invasive bacteria and viruses and presenting them to the body’s defense team, such as the T lymphocytes.
If the appendix shelters beneficial bacteria and boosts immune response, that may explain its links to various diseases. According to an epidemiological study from Taiwan,patients who underwent an appendectomy have a 46% higher risk of developing irritable bowel syndrome (IBS) — a disease associated with a low abundance of Butyricicoccus bacteria. This is why, the study authors wrote, doctors should pay careful attention to people who’ve had their appendixes removed, monitoring them for potential symptoms of IBS.
The same database helped uncover other connections between appendectomy and disease. For one, there was type 2 diabetes: Within 3 years of the surgery, patients under 30 had double the risk of developing this disorder. Then there was lupus: While those who underwent appendectomy generally had higher risk for this autoimmune disease, women were particularly affected.
The Contentious Connections
The most heated scientific discussion surrounds the links between the appendix and conditions such as Parkinson’s disease, ulcerative colitis, and colorectal cancer. A small 2019 study showed, for example, that appendectomy may improve symptoms of certain forms of ulcerative colitis that don’t respond to standard medical treatments. A third of patients improved after their appendix was removed, and 17% fully recovered.
Why? According to Dr. Parker, appendectomy may work for ulcerative colitis because it’s “a way of suppressing the immune system, especially in the lower intestinal areas.” A 2023 meta-analysis found that people who’d had their appendix removed before being diagnosed with ulcerative colitis were less likely to need their colon removed later on.
Such a procedure may have a serious side effect, however: Colorectal cancer. French scientists discovered that removing the appendix may reduce the numbers of certain immune cells called CD3+ and CD8+ T cells, causing a weakened immune surveillance. As a result, tumor cells might escape detection.
Yet the links between appendix removal and cancer are far from clear. A recent meta-analysis found that while people with appendectomies generally had a higher risk for colorectal cancer, for Europeans, these effects were insignificant. In fact, removal of the appendix actually protected European women from this particular form of cancer. For Parker, such mixed results may stem from the fact that treatments and populations vary widely. The issue “may depend on complex social and medical factors,” Dr. Parker said.
Things also appear complicated with Parkinson’s disease — another condition linked to the appendix. A large epidemiological study showed that appendectomy is associated with a lower risk for Parkinson’s disease and a delayed age of Parkinson’s onset. It also found that a normal appendix contains α-synuclein, a protein that may accumulate in the brain and contribute to the development of Parkinson’s. “Although α-synuclein is toxic when in the brain, it appears to be quite normal when present in the appendix,” said Luis Vitetta, PhD, MD, a clinical epidemiologist at the University of Sydney, Camperdown, Australia. Yet, not all studies find that removing the appendix lowers the risk for Parkinson’s. In fact, some show the opposite results.