Transforming growth factor beta 1 or TGFbeta1 is a polypeptide member of the transforming growth factor beta superfamily of cytokines. It is a secreted protein that performs many cellular functions, including the control of cell growth, cell proliferation, cell differentiation and apoptosis. TGFbeta1 was first identified in human platelets as a protein with a molecular mass of 25kD with a potential role in wound healing. It was later characterized as a large protein precursor (containing 390aa) that was proteolytically processed to produce a mature peptide of 112aa. TGFbeta1 plays an important role in controlling the immune system, and shows different activities on different types of cell, or cells at different developmental stages. Most immune cells (or leukocytes) secrete TGFbeta1. Some T cells (e.g. regulatory T cells) release TGFbeta1 to inhibit the actions of other T cells. Interleukin 1- and interleukin 2-dependent proliferation of activated T cells, and the activation of quiescent helper T cells and cytotoxic T cells is prevented by the activity of TGFbeta1. Similarly, TGFbeta1 can inhibit the secretion and activity of many other cytokines including interferon-gamma, tumor necrosisfactor-alpha (TNF-alpha) and various interleukins. It can also decrease the expression levels of cytokine receptors, such as the IL-2 receptor to down-regulate the activity of immune cells. However, TGFbeta1 can also increase the expression of certain cytokines in T cells and promote their proliferation, particularly if the cells are immature. TGFbeta1 has similar effects on B cells that also vary according to the differentiation state of the cell. It inhibits proliferation and apoptosis of B cells, and plays a role in controlling the expression of antibody, transferrin and MHC class II proteins on immature and mature B cells. The effects of TGFbeta1 on macrophages and monocytes are predominantly suppressive; this cytokine can inhibit the proliferation of these cells and prevent their production of reactive oxygen and nitrogen intermediates. However, as with other cell types, TGFbeta1 can also have the opposite effect on cells of myeloid origin. For example, TGFbeta1 acts as a chemoattractant, directing an immune response to some pathogens; macrophages and monocytes respond to low levels of TGFbeta1 in a chemotactic manner. Furthermore, the expression of monocytic cytokines and phagocytic killing by macrophages can be increased by the action of TGFbeta1.