A clinically defined disease states with a largely unknown morphological background. Acute mast cell activation (MCA) is commonly seen in allergic reactions and often leads to the clinical signs and symptoms of anaphylaxis. Severe or even life‐threatening MCA may occur when the burden of mast cells is high and/or these cells are in an hyperactivated state. Mastocytosis may be associated with mast cell activation syndrome (MCAS).