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
The immunoglobulin level, which is specifically elevated in autoimmune pancreatitis is which of the following? 
Select and explain. 
A. IgM, B. IgE, C. IgG4, D. IgG2
C: IgG4
Answer: C: IgG4 
Explanation: Serum IgG4 is elevated at least twofold higher than 135 mg/dL in those with autoimmune pancreatitis. Reference: Harrisons Principles of Internal Medicine, 18th Edition, Page 2644