The immune system relies on specialized “effector” T cells to fight off pathogens. However, in chronic infections such as cancer or HIV, the perpetual activation of these cells can turn them into “exhausted” T cells unable to continue fighting. Scientists have long wondered if and how nutrient preference impacts cell identity. Now, scientists at the Salk Institute and collaborators have discovered that a nutritional switch from acetate to citrate plays a key role in determining T cell fates, shi...