The wall of the small intestine contains several areas rich in lymphoid tissue. These are called Peyer’s patches; it is believed that they form an important part of the gut’s defensive system. In the young they are more numerous. They swell in people suffering from infections such as typhoid fever. The appendix, located at the junction of the small and large intestine, is also rich in leucocytes. It is not essential to the immune system, as demonstrated by the fact that many appendices are removed because of acute appendicitis without harmful consequences for the body’s defence system.