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': 'Rheumatoid factor is directed against \nSelect and explain. \nA. IgG, B. IgD, C. IgA, D. IgM', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Rheumatoid factor is IgM directed against Fc portions of IgG', 'role': 'assistant'}]