Increased colonic mucosal mast cells associated with severe watery diarrhea and microscopic colitis

Summary A patient with microscopic colitis, clearly identifiable by its histologic characteristics, is presented in whom the histology also revealed large numbers of mast cells. The patient was treated with H₁ antagonists with prompt resolution of the diarrhea. The histologic and therapeutic findings in this case suggest the diarrhea and inflammation of microscopic colitis may be mediated by the degranulation of excessive numbers of mast cells in the colon and small bowel.