This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
[{'content': 'Serotonin is mainly secreted by? \nSelect and explain. \nA. Mast cells, B. Platelets, C. Neutrophills, D. Macrophages', 'role': 'user'}, {'content': 'Answer: B \nExplanation: o Serotonin (5-hydroxy tryptamine) is present in platelets and enterochromaffin cells.\no It is also present in mast cells of rodents, but not humans.', 'role': 'assistant'}]