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The Gut: Immunity Connection

Approximately seventy percent of our immune system resides within the gastrointestinal tract. This is because the gut is the primary place where things from outside can enter the body. It is the part of the body where pathogens have the greatest chance of passing through the body’s outer barriers and gain entry into the system. It is the gate so to speak. As such, we post most of our immune defenses and defenders there.

Important components of the gut’s immune defenses and defenders are the physical barrier provided by the cells of the gut wall, the mucosal lining, and the microbial communities (microbiota) that reside there.

The cells that line the intestine are called epithelial cells. They provide a physical barrier preventing entry of pathogens. They also secrete mucin and proteins with antimicrobial properties. Epithelial cells also play a key role in transmitting chemical signals back and forth between the immune system and the microbiota in the gut.

Within the physical barrier of the gut lies the largest collection of immune cells in the body, the gut-associated lymphoid tissue (GALT). A primary function of the GALT is to identify, monitor and manage the gut microbiota and other antigens. As a result, the gut microbiota and human immune system constantly interact, with the immune system influencing the gut microbiota, and the gut microbiota influencing immune function.

Commensal bacteria that permanently reside in our guts are referred to as our microbiota. We have about 100 trillion bacteria in our gut. At birth, it is the growing population of friendly bacteria that guide and enhance the development of our immune system. In adulthood, our microbiota continue to support our immunity by communicating directly with our immune cells, and by producing anti-microbial substances that kill pathogenic bacteria. Additionally, the microbiota produce short chain fatty acids and other nutrients that feed the epithelial cells and help with regeneration and repair of the intestinal lining.

Our diet also provides essential nutrients for our immune system and microbiota. Vitamins A, D, C, E, B6, B12, folate, and zinc all play an important role in supporting immune function in the gut. Vitamin A contributes to the maintenance of structural and functional integrity of the gut’s physical immune barrier. Vitamin D plays an important role in the formation and maintenance of epithelial and endothelial cells that line the gut. Vitamin D also helps the immune system regulate microbial populations there. Vitamin C helps produce collagen which strengthens the epithelial cell barrier. Vitamin E helps maintain epithelial cell membranes and protects them from oxidative damage. Vitamin B6 mediates lymphocyte migration into the intestine, while folate is essential for the survival of regulatory T cells in the small intestine, and gut microbes use vitamin B12 as a cofactor for metabolic pathways, thus supporting the gut barrier. Last but not least, the body requires zinc to maintain the integrity of mucus membranes such as those that line the entirety of the digestive tract.

It can be argued that our gut is the immune system’s single most important organ. It is the operational center of our immune system. Keeping it healthy is of major importance. Eating right, staying physically active, spending time in the natural environment, and immune-specific supplementation is the recipe for a strong and properly functioning immune system.

I leave you this time, till we meet again, with a thought about another important role our guts play in our lives. Our futures, fortunate or troubled, can only be seen through a glass darkly. Yet we cannot remain still and expect good to come by chance. We first must act, and so we do, by the mysterious intuition of our gut, and faith in that which is greater than ourselves.

Yours in health,

-Dr. Bryan