What is MCAS?   Mast Cell Activation Syndrome is an abnormal immune response involving a subtype of white cells, called mast cells. These cells are located in connective tissue throughout the body, with higher concentrations near blood vessels and nerves, as well as at interfaces with the external environment, like the skin, lungs, and intestines. Mast cells act as guardians: when a virus or bacterium invades the body, they are activated and release around 1,200 different inf