KIT and systemic mastocytosis: Based on the etiology, MCAS can be divided into five variants: (1) primary/monoclonal MCAS (MMAS), where KIT-mutated, clonal MCs are detected, and it may either fulfill the criteria for mastocytosis (SM or CM) or present with only two minor SM criteria; (2) secondary MCAS where the disorders is triggered by an underlying IgE-dependent allergy or other immunologic disorder; (3) HαT MCAS; (4) combined MCAS where more than one of the above pathologies co-exist; and 5) idiopathic MCAS, in which none of the above causative triggers can be identified [30,31,32,33] (Table 1).