Research into the delicate and complex checks and balances that regulate the immune response is increasing knowledge of normal and abnormal immune functions. Someday it may be possible to treat diseases such as systemic lupus erythematosus by suppressing parts of the immune system that are overactive and stimulating those that are underactive. Blocking a mechanism such as co-stimulation, by paralyzing T cell responses, may make it possible to prevent graft rejection or to sideline autoimmune diseases, while boosting co-stimulation might help the body rid itself of cells that are infected or cancerous.