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
Which of the following is Cold agglutinin? 
Select and explain. 
A. IgG antibody, B. IgM antibody, C. Donath landsteiner antibody, D. IgA antibody
B: IgM antibody
Answer: B: IgM antibody 
Explanation: IgM cold-reacting antibodies readily agglutinate RBC and are called cold agglutins. Antibodies that react with polysaccharide antigens are usually IgM and bind antigen better at temperature lower than 37 degree celsius, hence the name cold-reactive antibodies. Uncommonly, the antibody is IgG (the Donath-Landsteiner antibody of paroxysmal cold hemoglobinuria). Ref:Harrison's Principles of Internal Medicine, 16th Edition, Pages 612-13