Tacrolimus, also known as fujimycin or FK506, is an immunosuppressive drug used mainly after allogeneic organ transplant to lower the risk of organ rejection. It achieves this by inhibiting the production of interleukin-2, a molecule that promotes the development and proliferation of T cells, which are vital to the body's learned (or adaptive) immune response. Tacrolimus is also used in the treatment of other T cell-mediated diseases such as eczema and psoriasis.