(1)Department of Medicine and Pediatrics, Nassau University Medical Center, 2201 Hempstead Turnpike, East Meadow, NY 11554, USA. mfrieri@numc.edu. Mast cell activation syndrome (MCAS) is a condition with signs and symptoms involving the skin, gastrointestinal, cardiovascular, respiratory, and neurologic systems.It can be classified into primary, secondary, and idiopathic.1)Department of Medicine and Pediatrics, Nassau University Medical Center, 2201 Hempstead Turnpike, East Meadow, NY 11554, USA. mfrieri@numc.edu. Mast cell activation syndrome (MCAS) is a condition with signs and symptoms involving the skin, gastrointestinal, cardiovascular, respiratory, and neurologic systems.