A surprising discovery about a failed pain drug -- and specifically, the pathway it targets, BH4 -- could have implications for autoimmunity and cancer. Neuroscientists report that BH4 also functions as a kind of immunological thermostat, raising and lowering the activity levels of T cells. Inhibiting BH4 could relieve atopic dermatitis, psoriasis, lupus, polyarthritis and inflammatory bowel disease; boosting it could help the immune system attack cancers.