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
Rheumatoid factor in Rheumatoid ahritis is: 
Select and explain. 
A. IgG, B. IgM, C. IgA, D. IgD
B: IgM
Answer: B: IgM 
Explanation: Answer is B (IgM): Rheumatoid ahritis is an IgM antibody directed against the Fe fragment of IgG.