[4] Although it has been long accepted that the immune tissue surrounding the appendix and elsewhere in the gut—called gut-associated lymphoid tissue—carries out a number of important functions, explanations were lacking for the distinctive shape of the appendix and its apparent lack of specific importance and function as judged by an absence of side effects following its removal.

[14][15] This proposition is based on an understanding that emerged by the early 2000s of how the immune system supports the growth of beneficial intestinal bacteria,[16][17] in combination with many well-known features of the appendix, including its architecture, its location just below the normal one-way flow of food and germs in the large intestine, and its association with copious amounts of immune tissue.

Research performed at Winthrop–University Hospital showed that individuals without an appendix were four times as likely to have a recurrence of Clostridioides difficile colitis.

[14] This reservoir of bacteria could then serve to repopulate the gut flora in the digestive system following a bout of dysentery or cholera or to boost it following a milder gastrointestinal illness.

This structure helps in the proper movement and removal of waste matter in the digestive system, contains lymphatic vessels that regulate pathogens, and lastly, might even produce early defences that prevent deadly diseases.

Additionally, it is thought that this may provide more immune defences from invading pathogens and getting the lymphatic system's B and T cells to fight the viruses and bacteria that infect that portion of the bowel and training them so that immune responses are targeted and more able to reliably and less dangerously fight off pathogens.

[24] Other signs and symptoms may include nausea and vomiting, low-grade fever that may get worse, constipation or diarrhea, abdominal bloating, or flatulence.

The appendix is also used for the construction of an efferent urinary conduit, in an operation known as the Mitrofanoff procedure,[26] in people with a neurogenic bladder.

The appendix is also used as a means to access the colon in children with paralysed bowels or major rectal sphincter problems.

The appendix is brought out to the skin surface and the child/parent can then attach a catheter and easily wash out the colon (via normal defaecation) using an appropriate solution.

[27] Charles Darwin suggested that the appendix was mainly used by earlier hominids for digesting fibrous vegetation, then evolved to take on a new purpose over time.

[28] Dr. Heather F. Smith of Midwestern University and colleagues explained: Recently ... improved understanding of gut immunity has merged with current thinking in biological and medical science, pointing to an apparent function of the mammalian cecal appendix as a safe-house for symbiotic gut microbes, preserving the flora during times of gastrointestinal infection in societies without modern medicine.

A cecal appendix has evolved independently at least twice and represents yet another example of convergence in morphology between Australian marsupials and placentals in the rest of the world.

Given that this organ may have a selective advantage in numerous situations, it appears to be associated with greater maximal longevity, for a given body mass.

Current epidemiological data on the cause of death in developing countries collected by the World Health Organization in 2001 show that acute diarrhea is now the fourth leading cause of disease-related death in developing countries (data summarized by the Bill and Melinda Gates Foundation).