What GALT is and where the '70%' figure comes from
Gut-associated lymphoid tissue (GALT) includes Peyer patches in the small intestine, immune cells scattered through the lamina propria, and the plasma cells that secrete secretory IgA into the gut lumen. The often-quoted claim that the gut holds about 70 percent of the body's immune tissue reflects the very large number of immune cells stationed along the digestive tract; the exact percentage is an estimate that varies by how tissue is counted, so it is best treated as 'a large share' rather than a precise constant.
Why so much immunity sits at the gut wall
The gut lining is the body's largest interface with the outside world, constantly exposed to food antigens and trillions of microbes. GALT must do two opposing jobs: tolerate harmless food and commensal bacteria while staying ready to respond to pathogens. Secretory IgA helps by coating and neutralizing microbes without provoking inflammation. This balance, and how diet and the microbiome influence it, is an active area of study.
This is general educational information, not medical advice.