Interleukin 1β (IL-1β) is a proinflammatory cytokine that is secreted mainly by monocytes and macrophages. IL-1β secretion has also been reported for a variety of other cells, including B cells, NK cells, dendritic cells, astrocytes, and microglial cells. It mediates inflammatory responses in B cells, T cells, and NK cells by inducing the production of cytokines, such as IL-2, IL-3, IL-6, as well as interferons. Upon exposure to IL-1β, endothelial cells and smooth muscle cells synthesize prostaglandins and other derivatives of arachidonic acid. In addition, IL-1β is found in synovial fluid of arthritis patients, causing degranulation of basophils and eosinophils as well as activation of osteoclasts. IL-1β is mitogenic for mesangial cells, glial cells, and keratinocytes. It has been shown that IL-1β is a potent modulator of CD40L-induced cytokine secretion by human dendritic cell (DC) subsets, such as monocyte-derived dendritic cells (MoDCs), CD34+-derived DCs, and peripheral blood DCs.