ASM A rare condition in which too many mast cells (a type of white blood cell) build up in certain tissues and organs in the body, including the bone marrow, lymph nodes, bone, liver, spleen, and small intestine. ASM may get worse quickly and cause severe symptoms and organ damage. Sometimes, it may become mast cell leukemia (a very rare type of leukemia) . ASM usually occurs in adults. Also called aggressive systemic mastocytosis.