Is the appendix actually useful for your gut microbiome?

The appendix contains a rich bacterial biofilm that continuously releases bacteria onto the intestinal wall. Researchers believe this small organ may therefore serve as a reservoir of healthy gut bacteria: after a severe episode of diarrhoea, the appendix could help recolonise the large intestine with beneficial flora. The bacterial diversity in the appendix is just as great as in the large intestine itself. This mechanism is biologically plausible, but has not yet been demonstrated in controlled studies in humans.

The appendix also has an immune function. The tissue is rich in lymphocytes, comparable to similar structures in the small intestine, and is an important production site for IgA. That is a type of antibody that regulates the composition and density of the gut microbiome. This function has been preserved through evolution, which suggests that the appendix is not simply a useless remnant.

People who have had their appendix removed have a somewhat different gut microbiome. Epidemiologically, there is an increased risk of disruptions to it (dysbiosis), and there are indications that recovery from certain intestinal conditions, such as recurring Clostridioides difficile infections, is more difficult without an appendix. These are associations; whether removal is the cause or whether other factors are involved has not been proven.

Remarkably, a series of larger cohort studies shows an opposite pattern for one specific condition: people without an appendix appear to have a lower risk of developing ulcerative colitis (a chronic bowel inflammation) and of subsequent flare-ups. Studies in patients with ulcerative colitis also found more activated immune cells in appendix tissue, suggesting that the appendix plays a role in the inflammatory response in that disease. Exactly how remains unclear, and the findings are not without controversy.

The link with colorectal cancer is, for now, unclear. Epidemiological signals have been reported, but the evidence is too limited to say anything meaningful about cause or effect.