Interleukins
Interleukins (ILs) were classically defined as cytokines that are secreted by one leukocyte to affect other leukocytes; however, it is now known that many cell types produce these mediators.
Some of the more well-known ILs are listed in Table 2.2. Over 30 ILs have been characterized.Table 2.2 Selected interleukins and their primary functions
| Interleukin | Primary function |
| IL-1 | Fever, T-cell, and macrophage (M1) activation |
| IL-6 | Acute phase protein production |
| IL-12 | IFNγ production, cell proliferation, innate lymphoid cell, and T-cell cytotoxicity |
| IL-2 | T-cell proliferation |
| IL-15 | T-cell proliferation |
| IL-10 | Suppresses M1 functions |
| IL-13 | Inhibits M1 cytokine production |
| TGFβ | Inhibits cell growth |
| IL-5 | Eosinophil chemotaxis |
| IL-8 (CXCL8) | Neutrophil chemotaxis |
Activated macrophages produce several key proinflammatory cytokines (Ferrante & Leibovich, 2012). For example, IL-1 stimulates the synthesis of acute phase proteins by the liver, causes the endothelium to express adhesion molecules and promote coagulation, stimulates neutrophil migration, affects the hypothalamus (producing fever), stimulates PG production and the respiratory burst of neutrophils, and acts synergistically with IFNγ and IL-2 to enhance inflammation and the immune response. IL-1 also stimulates the resorption of bone and cartilage by stimulating osteoclasts. This role of IL-1 may partially explain the hypercalcemia that is seen with some chronic granulomatous diseases. IL-1 is important for granuloma formation and enhances the size and persistence of these inflammatory lesions. Bacterial endotoxin is a very potent stimulator of IL-1. Like IL-1, IL-6 is a key mediator of acute phase protein synthesis by hepatocytes. It also triggers immunoglobulin synthesis by B lymphocytes. IL-12 is often released by macrophages with IL-1 and IL-6. This cytokine induces differentiation of naïve T lymphocytes into Th1 cells; stimulates the proliferation, maintenance, and activation of Th1 lymphocytes and cNK cells; stimulates the production of IFNy, IL-2, and TNF; and inhibits the development of Th2 cells.