Their primary function is to absorb nutrients and vitamins. The small intestine is made up of three parts—the duodenum (first part), the jejunum (middle part), and the ileum (end part).

Pathogen-fighting immune cells called tissue-resident memory CD8 T cells (TRM cells) go through a surprising transformation -- and relocation -- as they fight infections in the small intestine.

Surface area for drug absorption The villi and microvilli responsible for the vast absorptive surface in the small intestine are of highest concentration in the duodenum and proximal jejunum.