Figure 1.

The gastrointestinal (GI) tract contains varying frequencies and distributions of immune cells and vitamin-producing bacteria. The gut-associated lymphoid tissue (GALT) is essential to host immune defense and is comprised of mesenteric lymph nodes (MLNs), isolated lymphoid follicles (ILFs), and Peyer’s patches—found mainly in the small intestine. Gut microbes can express pathogen-associated molecular patterns (PAMPs) and immune cells recognize them through their pattern recognition receptors (PRRs). Antigen-presenting cells (APCs) undergo maturation upon PAMP-PRR interactions on their cell surfaces, which can then lead to the differentiation of naïve CD4⁺ T cells in the MLNs and outside the intestines and boost T- and B-cell-related responses. Th17 and Treg cells are the most abundant T cells in the gut and their concentration gradients along the tract share an inverse relationship. Bacterial abundances also vary along the GI tract, with the duodenum containing the lowest bacterial abundance which increases towards the colon, dominated by Firmicutes and Bacteroides in healthy individuals. Gut-derived vitamins are mainly generated and absorbed in the large intestine, while dietary vitamins are absorbed in the small intestine. Vitamin K2 and most members of the water-soluble B vitamins—thiamine, riboflavin, pantothenic acid, pyridoxine, biotin, folate, and cobalamin—are produced by gut bacteria and play many roles in immune function.