Mast cells are immune cells that are involved in allergic reactions such as hives, anaphylaxis, dermatographia, and sometimes intestinal inflammation. Mast cells can become activated by two things: allergens or autonomic nerves. Mast Cell Activation Syndrome (MCAS) is a condition defined by abnormal mast cell activation and allergic reactions - but without any obvious allergic cause. (For allergic reactions with a known cause, see the "Immunology, Inflammation, Allergies" forum.)