The main source of endogenous IL-2 is the T-cell. The targets are lymphocytes with T and B cell activation and NK cell activation. It has been used to treat a number of malignancies such as metastatic melanoma and renal cell carcinoma. However, its clinical usefulness appears to be limited by dose-dependent toxicity manifest as the vascular leak syndrome. It also initially increases the number of T cells which then fall dramatically. The vascular leak syndrome is thought to be due to IL-2 induction of leucocyte adherence and transmigration leading to a loss in endothelial barrier function.