Systemic mastocytosis is a blood disorder characterized by infiltration of clonally derived mast cells in different tissues, including bone marrow, the gastrointestinal (GI) tract, the liver, and the spleen. What is the Pathology of Systemic Mastocytosis? The pathology of systemic mastocytosis is: -Etiology: The cause of systemic mastocytosis is a random change (somatic mutation) in