Interleukins (ILs) are a group of cytokines that were first described in the 1970s. These proteins are secreted by a variety of cells and play important roles in the immune response. ILs are divided into four main groups: IL-1, IL-2, IL-6, and IL-12. IL-1 is a pro-inflammatory cytokine that is involved in the acute phase response. IL-1 is secreted by macrophages and dendritic cells in response to infection or tissue injury. IL-1 promotes the production of other pro-inflammatory cytokines, such as IL-6, and stimulates the release of histamine and other inflammatory mediators from mast cells. IL-1 also plays a role in the development of fever and the production of acute phase proteins by the liver. IL-2 is a cytokine that promotes the growth and differentiation of T cells. IL-2 is secreted by activated T cells and helps to maintain the immune response by stimulating the proliferation of T cells and the production of antibodies. IL-6 is a cytokine that has both pro-inflammatory and anti-inflammatory effects. IL-6 is secreted by a variety of cells, including macrophages, dendritic cells, and T cells. IL-6 promotes the production of other pro-inflammatory cytokines, such as IL-1 and TNF, and stimulates the release of acute-phase proteins by the liver. IL-6 also has anti-inflammatory effects and can inhibit the production of pro-inflammatory cytokines. IL-12 is a cytokine that is involved in the development of cell-mediated immunity. IL-12 is secreted by dendritic cells and macrophages and promotes the differentiation of naive T cells into Th1 cells. Th1 cells produce pro-inflammatory cytokines, such as IL-2 and IFN-gamma, and are important for the clearance of intracellular pathogens. The four main groups of interleukins are just a small part of the larger family of cytokines. There are many other interleukins that play important roles in the immune response, including IL-4, IL-10, IL-13, and IL-17.