Although much progress has been made in understanding the pathophysiology of inflammatory bowel disease, the precise etiology of Crohn's disease and ulcerative colitis remains unknown. It has been suggested that the complex interactions between an initiating factor and the gut immune system are primarily responsible for the promotion and perpetuation of the chronic inflammation associated with this disease. Because of this, much attention has been focused on studying the role of immunocytes and their inflammatory mediators in the pathogenesis of inflammatory bowel disease. These inflammatory mediators include 1) proinflammatory cytokines (interleukin-1, tumor necrosis factor-α, and interleukin-8), 2) inhibitory cytokines (interleukin-1 receptor antagonist), 3) immunoregulatory cytokines (interleukin-2, interleukin-6, and interferon-γ), 4) lipid mediators (prostaglandin E2, leukotriene B4, and platelet activating factor), 5) oxygen free radicals, and 6) neuropeptides.