The alimentary canal consists of the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus.
Natural defenses include mucus, secretory immunoglobulin A (IgA), fluids with antimicrobial properties, saliva, and the low pH of stomach fluid.
Millions of microorganisms are residents of the alimentary canal, where they provide digestive and immune services including training their host's immune system.