What function does the appendix actually serve?

The answer

The appendix was long considered a vestigial organ, but research has revealed its important immunological and microbiome functions. In 2007, Duke University researchers proposed that the appendix serves as a 'safe house' for beneficial gut bacteria, allowing the microbiome to reboot after severe infections like cholera or dysentery that flush the intestines. The appendix contains high concentrations of lymphoid tissue and immunoglobulin A-producing immune cells, functioning as part of the gut-associated lymphoid tissue (GALT). A 2017 study analyzing 533 mammalian species found the appendix evolved independently at least 30 times across different lineages - strong evidence of functional importance, since useless structures tend to disappear. People who have had appendectomies show a 19.7% increased risk of Clostridium difficile infection. The appendix also produces specific antibodies and trains immune cells during fetal development.