What does the drug tacrolimus do?

Tacrolimus works by stopping the activity of an enzyme (protein) in the body called calcineurin. Calcineurin plays a key role in activating T cells — white blood cells that help regulate immune responses. When an organ is transplanted into a new body, the recipient's immune system may recognize it as foreign and attack it. By blocking calcineurin, tacrolimus can make the immune system less active, making organ rejection less likely.