In the intestine, a unique immunological system that is different from the systemic immune system exists to provide adaptive immunity in response to luminal bacteria and dietary antigens. There are many lymphoid cell aggregates called gut-associated lymphoid tissue (GALT) including Peyer's patches (PPs), which function as important induction sites for the mucosal immune response. M-cells are present in the epithelium of PPs, having a specialized structure for uptake of macromolecules such as bacteria. In addition to GALT, there are abundant lymphoid cells in the intestinal lamina propria,...