Ulcerative colitis (UC) is an inflammatory disease of the rectal and colonic mucosa and seems to result from a complex series of interactions between susceptibility genes, the environment and the immune system. Various components of the mucosal immune system are implicated in the immunopathogenesis of UC. Evidence from animal models also suggests that an altered immune response to the commensal microflora of the host plays a central role in the development of UC. So in this review, we elucidate the cells and molecules which are implicated in the immunopathogenesis of the disease from four aspects: antigens in the intestine, dendritic cells, toll like receptors and NF-κB in the UC.