MCAS is sub-divided in primary or clonal MCAS in which a clonal MC disease is identified, the KIT D816V mutation is detected and MCs aberrantly express CD25 in most cases; secondary MCAS where an IgE-mediated allergy or another hypersensitivity/immunologic reaction that can induce MCA is diagnosed, in absence of neoplastic MCs or KIT D816V mutation. The gene discussed is IGHE; the disease is Allergy.