Cytokine secreted predominantly by activated T-lymphocytes as well as mast cells and osteoblastic cells that controls the production and differentiation of hematopoietic progenitor cells into lineage-restricted cells. Stimulates also mature basophils, eosinophils, and monocytes to become functionally activated. In addition, plays an important role in neural cell proliferation and survival. Participates as well in bone homeostasis and inhibits osteoclast differentiation by preventing NF-kappa-B nuclear translocation and activation. Mechanistically, exerts its biological effects through a receptor composed of IL3RA subunit and a signal transducing subunit IL3RB (By similarity). Receptor stimulation results in the rapid activation of JAK2 kinase activity leading to STAT5-mediated transcriptional program (PubMed:8378315, PubMed:10376805). Alternatively, contributes to cell survival under oxidative stress in non-hematopoietic systems by activating pathways mediated by PI3K/AKT and ERK (By similarity). Monomer. Activated T-cells, mast cells, natural killer cells. IL-3-deficient animals show no abnormalities. Analysis of steady-state hematopoiesis demonstrates normal numbers of peripheral blood cells, bone marrow and splenic hematopoietic progenitors. However, they show impaired contact hypersensitivity reactions. Belongs to the IL-3 family.