Transgenic mice that constitutively over express IL‐5 (IL5T [IL‐5 transgenic]: IL‐5 is produced by T cells, under control of the CD2 promoter) have a permanent and extreme eosinophilia, where eosinophils make up over half of all leukocytes in the BM.3 IL‐5 acts at multiple levels throughout the eosinophil lineage, regulating eosinophil production, activation, migration, and survival.3, 4, 5, 6. This evidence concerns the gene IL5 and Increased total eosinophil count.