What Does the Appendix Do?

The human appendix is a small, muscular, vermiform or worm-shaped organ, averaging around 9cm in length, that is located at the junction of the small and large intestine.¹

Until recently, scientists regarded the appendix to be a useless organ in humans. Berengario da Carpi first described the human appendix in 1521.² The pathology of this organ triggered significant interest for centuries because of its notorious tendency to become inflamed, causing a condition called appendicitis that results in considerable pain in the lower right abdomen. Clinicians diagnose appendicitis by physical examination, laboratory evaluation, and abdominal imaging by ultrasound and x-ray.³ Although the diagnosis and treatment of a diseased appendix have improved significantly, its precise role in the human body and evolutionary significance still provoke curiosity and controversy.

Is the Appendix a Vestigial Organ?

According to comparative anatomy researchers, some organs are classified as vestigial because they may not presently have a function within the body. These organs may have had significant roles in the past, which are now lost due to evolutionary changes. 

Researchers believed the human appendix to be a vestigial organ based on evolutionary evidence of its anatomical and functional changes in different species.⁴ For instance, the mammalian appendix is significantly longer in herbivores than in carnivores and omnivores, and it contains cellulose-digesting bacteria that aid in digestion. The small size or absence of an appendix in carnivorous or omnivorous animals indicated its relatively minor physiological role in their digestive systems.⁵ Furthermore, the removal of the appendix in humans did not seem to cause any harm, lending credence to the belief that it was a vestigial organ. Recent research findings question the vestigial status of the appendix, instigating researchers to continue exploring its potential physiological roles.⁵

Why Do We Have an Appendix?

Recent studies have raised the possibility that the appendix may play a role in gut microbiome restoration and immunological functions. 

Gut microbiome restoration

Researchers hypothesized that the human appendix is a “nature reserve” for beneficial bacteria.⁶ Anatomically, the appendix pouch extends from the cecum to the large intestine, and this architecture enables the human appendix to be a “safe house” for commensal bacteria. “It's a blind organ, where microbes could escape different insults to the gut physiology such as infection, or antibiotics, or toxins," explained Eytan Wine, a pediatric gastroenterologist at the University of Alberta, Canada. 

Colon infection by bacterial pathogens such as Clostridium difficile cause microbial dysbiosis. Recent studies suggested that after such pathogenic exposure, biofilms within the appendix may facilitate re-inoculation by shedding healthy microbes that restores its normal flora.⁶ 

Immunological functions

Researchers also suggested that re-inoculation of commensal bacteria by the appendix helps maintain human immunity.⁵ “Our immune system is taking advantage of whatever's in the safe house to try and study, prepare, and react to our potential invaders in a way that is usually beneficial,” said Wine. Therefore, after an appendectomy, “you lose the safe house, but you also lose that immune education memory, possibly, that might be offering us protection.”

Another study showed that the presence of microbial flora in the rabbit appendix is critical for gut-associated lymphoid tissue (GALT) development and somatic diversification, which in turn establishes the primary antibody repertoire.⁷Other studies highlighted that appendectomy is also associated with increased or decreased risk for later development of inflammatory diseases such as rheumatoid arthritis and ulcerative colitis, indicating the possible immunological function for the appendix.^8,9 

Longevity in mammals

Evolutionary studies unveiled that the appendix evolved multiple times , which could be associated with greater longevity across mammalian species.¹⁰ Researchers correlated the increased longevity with reduced exposure to harmful pathogens. For example, it is possible that beneficial bacteria present in the appendix may decrease the risk of infections causing fatal diarrheal diseases, particularly in young children. 

Appendicitis: Symptoms and Treatment

Anatomically, the appendix has low storage capacity and secretes around 2-3mL of mucus daily. Mucilage accumulation occurs when a stony mass, called fecalith, blocks the narrow lumen of the appendix. This blockage and subsequent mucilage accumulation triggers inflammation of the appendix causing appendicitis.³ 

Acute appendicitis is the most common pathology of the appendix, causing sudden pain in the lower right abdomen, nausea, anorexia, and vomiting. Sometimes, an inflamed appendix ruptures and releases its contents into the abdominal cavity. A person with a ruptured appendix may develop sudden abdominal pain, fever, and weakness. This is an extremely serious condition that could lead to death.

Besides appendicitis, other appendix-related pathologies include congenital anomalies (e.g., complete absence of appendix or appendix duplications) and appendiceal tumors. These conditions may trigger symptoms similar to acute appendicitis and lack specific symptoms.

Laparoscopic appendectomy is the most common treatment that entails the removal of an inflamed appendix. Physicians use broad-spectrum antibiotic monotherapy such as piperacillin-tazobactam, or combination therapy with either fluroquinolones or cephalosporins and metronidazole to treat uncomplicated acute appendicitis.¹¹

FAQ

Do humans need an appendix?

• Although people live healthy lives without an appendix, recent studies have indicated its key functional roles, such as providing immunity and gut microbiome maintenance.

Where is the appendix located in the human body?

• The human appendix is a cylindrical muscular structure present near the junction of the cecum and ileum. Although in most people it is present at the lower right side of the abdomen, in a rare condition known as situs inversus, it appears at the lower left quadrant of the abdomen.¹²

How many times has the appendix evolved?

• There exists considerable uncertainty about the exact number of times that the appendix has evolved. While one cladistic analysis revealed that the cecal appendix evolved independently at least twice, another phylogenetic study estimated that the appendix evolved independently at least 29 times throughout mammalian evolution and was lost a maximum of 12 times.^13,14

Do other animals have an appendix?

• Yes, many animals have an appendix. Mammals within various clades such as Afrotheria, Marsupialia,Monotremata, and Euarchontoglires possess an appendix.¹⁵

What is the function of the appendix in digestion?

• The appendix may serve as a reservoir of beneficial gut bacteria. In the event of gut microbial dysbiosis, due to intestinal infection or antibiotic treatment, it may restore the gut microbial balance via reinoculation.

What is the function of the appendix in the immune system?

• The appendix is rich in lymphoid tissue and supports intestinal immunity.⁵ It assists in immunoglobulin A production and B lymphocytes maturation.