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
Rheumatoid factor is directed against 
Select and explain. 
A. IgG, B. IgD, C. IgA, D. IgM
A: IgG
Answer: A: IgG 
Explanation: Rheumatoid factor is IgM directed against Fc portions of IgG