Human IL-6 is active on both mouse and rat cells, while mouse IL-6 has no activity on human cells. Cells known to express IL-6 include CD8+ T cells, fibroblasts, synoviocytes, adipocytes, osteoblasts, megakaryocytes, endothelial cells (under the influence of endothelins), sympathetic neurons, cerebral cortex neurons, chromaffin cells of the adrenal medulla, retinal pigment cells, mast cells, keratinocytes, Langerhans cells, fetal and adult astrocytes, neutrophils, monocytes, eosinophils, colonic epithelial cells, B1 B cells, and most likely, pancreatic islet beta cells. IL-6 production is generally correlated with cell activation. Circulating IL-6 can be found in the blood of normal individuals in the 1 pg/mL range, with slight elevations during the menstrual cycle, modest elevations in certain cancers (melanoma) (10 pg/mL), and large elevations after surgery (30-430 pg/mL).