Appendix Purpose

Appendix Purpose

Scientists have a new theory about the function of the appendix.


A job description for the appendix. I'm Bob Hirshon and this is Science Update.

Scientists have long wondered what the human appendix does. Now, Duke University immunologist William Parker and his colleagues have proposed a theory: that it's a safe house for the helpful bacteria in our gut. They've found that the appendix is especially rich in these good bacteria. And the organ's thin, narrow shape may shut out harmful infections.


Suppose some kind of amoeba gets in there that causes amoebic dysentery: something really bad. And basically your normal gut bacteria are all contaminated. Everything is going to get flushed out as a defensive mechanism.

Except, perhaps, from the appendix, which would then replenish the good bacteria from its protected reserves. Parker says such severe infections are rare in industrialized nations, which may explain why we can live normally with our appendix removed. I'm Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

The appendix is a two-to-four-inch-long, finger-like tube that dangles from the junction of the small and large intestine. For the past century or so, the appendix has seemed like nothing but trouble—at least in industrialized countries. That's because it can become inflamed and swollen (a condition called appendicitis), eventually rupturing and leaking toxic bacteria into the abdomen. In the early 20th century, appendicitis killed tens of thousands of Americans every year; even today, with much improved medical interventions, it still claims a few hundred lives.

To treat appendicitis, surgeons usually remove the appendix. The good news is that people can live perfectly normally without an appendix. But if that's the case, then what does the appendix do? That question has stumped scientists for a very long time, and since Darwin, the dominant answer has been that the appendix served a purpose to our prehistoric ancestors, but became obsolete long ago. (Darwin himself thought that the appendix may have helped our primate predecessors digest tough plant tissue.)

Parker and his colleagues, including R. Randal Bollinger, propose something a little bit different: Yes, the organ doesn't really serve a purpose to modern Americans—but the organ probably became obsolete within the past century or two, not over hundreds of thousands of years. What's more, their theory suggests that the appendix remains important even today, to people in certain parts of the world.

Parker himself admits there's not a “smoking gun” that proves his theory right, but several lines of circumstantial evidence support it. Over the past decade, his team has studied something called biofilm: a mash-up of normal, digestive bacteria, immune molecules, and sticky mucus that lines the gut. Their research suggests that certain immune molecules nourish and protect the good bacteria, by producing the mucus that gives them a food source and a stable place to grow. The good bacteria, in turn, crowd out harmful bacteria that might invade our system and make us sick. Parker's team has also found that the biofilm is especially rich and thick in the appendix, and thins out the further you get from the organ.

Furthermore, according to Parker, the shape of the appendix is especially well suited for keeping out bad bacteria: partly because it's thin and narrow, and partly because there's a constant outflow of good bacteria that would force harmful bacteria to “swim against the current” to infect the appendix. Other recent research has indicated that the appendix serves some kind of immune function.

From all of this, Parker's team concludes that the appendix would be able to hang on to a “starter set” of good bacteria even if the rest of the gut becomes so infected that the body flushes it out (through diarrhea and vomiting). Once the gut was emptied, new colonies of good bacteria would start to grow from the reserves in the appendix. In other words, it's a sort of Noah's Ark for the digestive system. In today's industrialized countries, however, good sanitation and clean water have made dangerous parasites and other severe intestinal illnesses rare, which explains why it makes little difference to take the appendix out.

In fact, modern sanitation may explain not only why we can live without our appendix, but why we sometimes need to have it removed. According to a popular hypothesis called the “hygiene hypothesis,” we've eliminated so many microbial threats with our clean water, antiseptics, and antibiotics, that our immune systems don't have enough to do. So they either attack harmless things, like pollen or peanuts (causing allergies and asthma), or our own bodies, causing auto-immune disorders including multiple sclerosis, arthritis, and lupus. Since the appendix contains a lot of immune tissue, it too may sometimes get inflamed because it's not active enough, leading to appendicitis and a trip to the emergency room.

Now try and answer these questions:

  1. What is the function of the appendix, according to this research team?
  2. What are some lines of evidence supporting their theory?
  3. What kind of evidence would it take to prove whether or not they're correct? Would it be possible to get this kind of evidence using ethical science experiments?
  4. What is the “hygiene hypothesis"? How does it relate to the role of the appendix, as described by Parker's team?

You may want to check out the November 2, 2007, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: How spiders avoid getting caught, plastic that's as strong as steel, a purpose for the appendix, and more.

For Educators

The Immune System: Friend and Foe, from the Howard Hughes Medical Institute's BioInteractive series, features an interactive presentation about the human immune system.

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