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: Medicine
[{'content': 'Rheumatoid factor in Rheumatoid ahritis is: \nSelect and explain. \nA. IgG, B. IgM, C. IgA, D. IgD', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Answer is B (IgM): Rheumatoid ahritis is an IgM antibody directed against the Fe fragment of IgG.', 'role': 'assistant'}]