The appendix, notorious for its tendency to become inflamed or even rupture, has historically been viewed as a vestigial organ with no real function. But new research supports the idea that the appendix may indeed serve a purpose: to protect beneficial bacteria living in the gut.

Heather F. Smith, PhD, associate professor at Midwestern University Arizona College of Osteopathic Medicine, has studied the evolution of gastrointestinal traits across different animal species. Her new research, published in the journal Comptes Rendus Palevol, analyzed the presence or absence of an appendix in 533 different mammals.

She found that the appendix evolved independently in different genetic “trees,” more than 30 separate times. Furthermore, the appendix almost never disappeared from a lineage once it appeared. This suggests that the organ remains for a reason, she says—an adaptive purpose.

Smith and her co-authors—from Duke University Medical Center, the University of Stellenbosch in South Africa and the Muséum National d’Histoire Naturelle in France—were able to reject several previous hypotheses that the appendix may be linked to dietary or environmental factors. But they did make one interesting discovery: Species who had an appendix tend to have higher concentrations of lymphoid tissue in their cecum, a pouch that connects the small and large intestine.

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This type of tissue can play a role in immunity, and can also stimulate growth of healthy gut bacteria. So it makes sense, Smith says, that the appendix actually serves as a “safe house” for these beneficial bugs.

This study isn’t the first to suggest that the appendix may play this type of role. The “safe house” idea was first raised by a 2007 study, which inspired Smith to question whether the appendix had evolved to serve this function in humans and other mammals—a theory that now appears quite likely.

So what does this mean for people who have had their appendix removed? Luckily, not much. “In general, people who have had an appendectomy tend to be relatively healthy and not have any major detrimental effects,” Smith says. (She herself had hers out at age 12.)

Some studies have shown, however, that people without an appendix may have slightly higher rates of infection than those with a functioning organ. “It may also take them slightly longer to recover from illness, especially those in which the beneficial gut bacteria has been flushed out of the body,” Smith added.

In a broader sense, Smith says that research on the appendix has provided “another line of evidence against over-sanitizing and excessive hygiene.” Because this organ is full of immune tissue, she says, one of the leading causes of appendicitis has to do with poorly developed immunity.

“Exposure to pathogens and infectious agents, like bacteria and viruses, is important for the normal development processes of the immune system,” Smith says. Without this exposure, development can be suppressed and the immune system can become hypersensitive—a hypothesis often used to explain illnesses like asthma and allergies.

More research in this area may help doctors address the organ’s most well known problem. “As treatments are developed for other autoimmune disorders and responses, it’s certainly possible that something similar may be developed for treating appendicitis,” she says.

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