Any of a class of cytokines that act to stimulate, regulate, or modulate lymphocytes such as T cells. Interleukin-1, which has two subtypes, is released by macrophages and certain other cells, and regulates cell-mediated and humoral immunity. It induces the production of interleukin-2 by helper T cells and also acts as a pyrogen. Interleukin-2 stimulates the proliferation of helper T cells, stimulates B cell growth and differentiation, and has been used experimentally to treat cancer. Interleukin-3 is released by mast cells and helper T cells in response to an antigen and stimulates the growth of blood stem cells and lymphoid cells such as macrophages and mast cells. There are many other interleukins that are part of the immune system.