Any of numerous hormone-like, low-molecular-weight proteins, chemically identified as purines, secreted by various cell types that regulate the intensity and duration of immune response and mediate cell-cell communication, as well as acting as signals in cell survival, growth, differentiation and apoptosis. Examples include interferon, interleukin, lymphokine, chemokines and various growth factors.
See Apoptosis (or cell death), Differentiation (embryology), Fibroblast growth factor (FGF), Growth factors, Polarizer, Proteins, Purines