Abstract: Insulin-dependent diabetes mellitus (IDDM), also known as type 1 diabetes, is an organ-specific autoimmune disease resulting from the destruction of insulin-producing pancreatic β cells. The hypothesis that IDDM is an autoimmune disease has been considerably strengthened by the study of animal models such as the BioBreeding (BB) rat and the nonobese diabetic (NOD) mouse, both of which spontaneously develop a diabetic syndrome similar to human IDDM. β cell autoantigens, macrophages, dendritic cells, B lymphocytes, and T cells have been shown to be involved in the pathogenesis of autoimmune diabetes. Among the β cell autoantigens identified, glutamic acid decarboxylase (GAD) has been extensively studied and is the best characterized. β cell-specific suppression of GAD expression in NOD mice results in the prevention of IDDM. Macrophages and/or dendritic cells are the first cell types to infiltrate the pancreatic islets. Macrophages play an essential role in the development and activation of β cell-cytotoxic T cells. B lymphocytes play a role as antigen-presenting cells, and T cells have been shown to play a critical role as final effectors that kill β cells. Cytokines secreted by immunocytes, including macrophages and T cells, may regulate the direction of the immune response toward Th1 or Th2 as well as cytotoxic effector cell or suppressor cell dominance. β cells are destroyed by apoptosis through Fas-Fas ligand and TNF-TNF receptor interactions and by granzymes and perforin released from cytotoxic effector T cells. Therefore, the activated macrophages and T cells, and cytokines secreted from these immunocytes, act synergistically to destroy β cells, resulting in the development of autoimmune IDDM.