Is the appendix actually useful for your gut microbiome?

The appendix harbours a rich collection of commensal gut bacteria, at least as diverse as those found elsewhere in the large intestine. Reviews describe how protective bacterial biofilms attach to the inner wall and are sheltered there. After a severe diarrhoeal infection that largely flushes out the large intestine, the appendix is thought to act as a reservoir from which healthy bacteria can repopulate the gut. This mechanism is biologically well-supported, but has never been directly tested in controlled studies in humans.

Beyond this reservoir function, the appendix is also an active immune organ. It contains a large amount of lymphoid tissue and is an important production site for IgA, an antibody that helps determine which bacteria thrive in the gut and in what quantities. Immune cells that help maintain the balance between the gut and its bacteria also mature there. This makes the appendix less of a passive 'pouch' than was long believed.

The evolutionary story also suggests the appendix is not there for nothing: it has evolved independently multiple times in mammals and rarely disappears once it is present. Congenital absence is rare. This points to some advantage, although it does not prove a specific function.

People without an appendix generally do perfectly well in wealthy countries, so the organ is not a survival necessity in a modern environment with clean drinking water. Nevertheless, epidemiological studies show an association between removal of the appendix and a higher risk of certain bowel diseases, such as ulcerative colitis, and a worse outcome in recurrent gut infections with the bacterium Clostridioides difficile. Whether that association is causal has not been proven. One caveat: biofilms elsewhere in the large intestine are not necessarily healthy. Thick bacterial layers in other parts of the gut are actually linked to bowel disease and colorectal cancer. Only in the appendix does this film appear to play a protective role.