Mastocytosis (say “mas-toe-sigh-toe-sis”) is a rare disease of the skin (the most common form), or of other parts of the body (very rare), like the stomach, the intestines and the bone marrow.It's caused by having too many mast cells. Mast cells are a kind of blood cell.Mastocytosis can occur at any age.astocytosis (say “mas-toe-sigh-toe-sis”) is a rare disease of the skin (the most common form), or of other parts of the body (very rare), like the stomach, the intestines and the bone marrow.