The development and function of eosinophils are regulated by a number of cytokines. Three cytokines have major effects on eosinophilopoiesis. Both granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 stimulate the development of eosinophils as well as other leukocytes. Interleukin-5 promotes eosinophil development and terminal differentiation. These three cytokines also effect the functions of mature eosinophils and can prolong their longevity in in vitro culture, enhance their capacity for release of leukotriene C4 (LTC4), augment their capacity for helminthotoxicity and degranulation, and render them less dense ("hypodense") than normal, unactivated eosinophils. GM-CSF can also induce the expression of HLA-DR on mature eosinophils, which can enable eosinophils to serve as antigen-presenting cells in stimulating T-cell responses. A T-cell-derived cytokine, lymphocyte chemoattractant factor (LCF), which stimulates the migration and function of CD4+ lymphocytes and eosinophils, also utilizes CD4 expressed on human eosinophils as its receptor. LCF stimulates eosinophil migration but not degranulation, leukotriene C4 release, or respiratory burst activity. Interleukin-2 is also a potent chemoattractant for eosinophils. Thus, cytokines are involved in both increased production of eosinophils as well as regulation of the functions of mature eosinophils. These functions of mature eosinophils include effector functions and collaborative interactions with lymphocytes and other tissue cellular elements.