A human cytokine is any member of a large class of proteins and peptides that carries signals between the cells of a living organism in order to control the organism's immune response to infection. Aside from their essential importance to the immune system, cytokines also play a role in other biological processes as well, such as the formation of embryos. The dividing line between cytokines and hormones is fuzzy due to their structural similarities. Cytokines come from cells distributed throughout the body, whereas hormones are primarily produced by specialized glands. Human cytokine has equivalents or close analogs in other vertebrates.
The immune systems of humans and other vertebrates produce many different types of cytokines that serve different functions in the body's immune response. Different kinds of human cytokine can deliver signals to other cells directing them to produce antibodies, regulate inflammation, and attack viruses, foreign microorganisms, and fellow body cells that have been damaged or infected by intruders. They are primarily produced by leukocytes, also known as white blood cells.
Interferons are a type of cytokine important in cellular responses to infection. When a type of lymphocyte called a T helper cell identifies foreign intruders in the body, it releases interferons that signal other immune cells, such macrophages, cytotoxic T cells (or killer T cells), and natural killer (NK) cells. These cells then protect the body by attacking and destroying intruding pathogens as well as fellow body cells that are infected or malfunctioning, such as those in tumors. Cells dying from viral infection also release interferons that warn other cells of the danger, causing them to produce other enzymes and proteins that interfere with viral reproduction.
Cytokines that are able to chemically influence the movement of other cells, a process called chemotaxis, are called chemokines. Chemokines aid the body's immune response by guiding immune system cells to where they are needed. Some chemokines are produced in the normal course of an organism's activities to regulate the movements of white blood cells as they patrol the body. Others are produced in large amounts when an infection is detected to summon white blood cells to the site of the potential danger.
Tumor necrosis factors (TNF) are cytokines that kill the body's own cells. They are released to destroy cells cells that have been taken over by viruses or become cancerous. TNF are also involved in the inflammation process.
Many types of human cytokine are part of a group called interleukins, which have a diverse array of functions. Different kinds of interleukin are commonly designated by the letters IL and a number. Some interleukins signal for the production of antibodies or the activation of immune cells such as macrophages to destroy pathogens, while others are important for triggering and regulating immune responses such as inflammation and fever. Interleukins are also important in immunological memory, the immune system's ability to adapt itself to more effectively fight off pathogens it has encountered in the past.