IGHE and systemic mastocytosis: There are also conditions that predispose individuals to MCA events and thus to MCAS, including hereditary alpha tryptasemia (HAT) and/or systemic mastocytosis (SM), which also can result in more severe IgE-dependent reactions, particularly to insect venoms (Supplemental Table S2) [21,22,23,40,41,42,43,44,45,46,47,48].