Answers: 1. When a mast cell tumor is physically manipulated during an exam, degranulation occurs. This triggers the release of histamine, which causes vasodilatation, erythema and hive formation. 2. This is called "Darier's sign." 3. The name comes from the French dermatologist Ferdinand-Jean Darier (1856–1938) who first described it. How did you do? Let us know in the comments below.