A large part of our immune system is closely associated with our digestive sytem.
Gut-associated lymphoid tissue (GALT) is indeed a major part of our immune system.
As part of the Mucosa-associated lymphoid tissue (MALT) it hosts lymphocytes, macrophages and other cells that participate in immune responses. Considering that our digestive system is laden with bacteria, it makes sense that it is under strict supervision by the immune system. The Gastrointestinal Immune System