Crohn's disease and ulcerative colitis represent two distinct forms of inflammatory bowel diseases (IBD). In this paper, we discuss how immunological mechanisms contribute to the pathogenesis of IBD. Intestinal homeostasis is sustained by various kinds of cells, such as epithelial cells, lymphocytes, antigen presenting cells, and other innate immune cells. We pay special attention to intestinal CD14+ macrophages. Intestinal macrophages play a central role in the regulation of immune responses against commensal bacteria. In the physiological condition, intestinal macrophages lack the expression of innate-immune receptor CD14 and do not produce proinflammatory cytokines. We identified a unique macrophage subset of IBD in the human intestine, which expressed both macrophage (CD14, CD33, CD68) and dendritic cell (DC) markers (CD205, CD209) and produced larger amounts of proinflammatory cytokines, such as interleukin (IL)-23 and tumor necrosis factor (TNF)-α. In addition, the CD14+ macrophages contributed to interferon (IFN)-γ production rather than IL-17 production by lamina propria mononuclear cells dependent on IL-23. We discuss herein this IL-23/IFN-γ-positive feedback loop in IBD patients. We also discuss IFN-γ and IL-17 production from mucosal T cells and natural killer (NK) cells. Here, we show our recent findings about the plasticity of T helper cells in colitis. Th 17 cells express T-bet, and finally lose the expression of retinoic acid-related orphan receptor (ROR)γt, the master regulator of Th 17 cells, and are differentiated 'alternative Th 1 cells.' In addition to Th 1 cells, mucosal NK cells are also important sources of IFN-γ. Some of our ideas may be provocative, but we hope this review paper will provide new and firm understanding of the pathogenesis of IBD.