Angiotensin-converting enzyme (ACE) is a protein that plays a key role in regulating blood pressure. It is found on the inner lining of blood vessels, with particularly high concentrations in the lungs. ACE is also produced by inflammatory cells involved in the body's immune response. Measuring ACE levels can be helpful in diagnosing and monitoring sarcoidosis, a disease of unknown origin that is characterized by the formation of granulomas—small clusters of immune cells and fibrous tissue that can affect various organs.