Mast Cell Activation Syndrome (MCAS) is a chronic immune disorder in which the body’s mast cells—a type of white blood cell—release chemical mediators inappropriately or excessively.

These mediators, including histamine, prostaglandins, and cytokines, play crucial roles in allergic and inflammatory responses. In MCAS, their release happens at the wrong time, in excessive amounts, or in response to triggers that normally wouldn’t cause a reaction.