Appendicitis is an infection of a small section of the bowel called the appendix. It is a pouch at the junction of the small and large intestines. The size of the small finger, it is lined with a mucous membrane that produces a clear secretion. This organ has no known function. One theory, though, is that it plays a role in the immune system very early in life.
Partly digested food and liquids traveling through the bowel pass in and out of the pouch when it is open. If this flow is blocked, bacteria trapped in the appendix may multiply. This is thought to cause appendicitis.