Why so much immune tissue lines the gut
The intestinal lining faces an enormous surface area exposed to food antigens and trillions of resident microbes. A large share of the body's immune cells sits in or near this tissue, organized as gut-associated lymphoid tissue (GALT). The figure of roughly 70 percent is widely cited as an approximation for the proportion of immune cells associated with the gut; exact numbers vary by definition and source.
The central challenge here is tolerance: the system must stay quiet toward harmless food and commensal bacteria while remaining ready to react to genuine pathogens.
How antigen sampling and secretory IgA work
Peyer patches are organized lymphoid follicles in the small intestine. Specialized M cells overlying them transport samples of luminal contents across the epithelium to immune cells waiting beneath, so the system can survey what is in the gut without breaching the barrier.
Plasma cells in the lining produce secretory IgA, which is exported into the lumen where it binds microbes and toxins and limits their attachment to the epithelium without provoking strong inflammation. This balance of sampling and IgA coating supports tolerance. This is general educational information about gut immunology, not medical advice.