This group of soluble molecules plays an extremely important role in clinical immunology. They are secreted by macrophages and may act as stimulatory or inhibi-tory signals between cells. Cytokines that initiate chemotaxis of leucocytes are called chemokines.
Among the group of cytokines, there are a few of particular interest because of their stimulatory activity. Interleu-kins 1 (IL-1) and 2 (IL-2) are of particu-lar importance secondary to their role in amplifying the immune response. IL-1 acts on a wide range of cells including T
and B cells. In contrast, IL-2 primarily acts on lymphocytes, although it has similar trophic effects on IL-receptor B cells and natural killer (NK) cells. (See Table 1.1 and functions.)