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: Microbiology
IgM have how many four-peptide subunits: March 2010 
Select and explain. 
A. 3, B. 4, C. 5, D. 6
C: 5
Answer: C: 5 
Explanation: IgM molecules are polymers of five four-peptide subunits. In the pentameric form all heavy chains are identical and all light chains are identical. It has another protein covalently bound a S-S bond called the J chain. This chain functions in polymerization of the molecule into a pentamer