This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: miscellaneous, Subject name: Biochemistry
True about IgM 
Select and explain. 
A. Fix Complement, B. Increased in primary response, C. Fab region is composed of variable region, D. Fc is antibody - binding option
A: Fix Complement
Answer: A: Fix Complement 
Explanation: IgM Produced in the primary response to an antigen Fixes complement.Does not cross the placentaAntigen receptors are on the surface of B cells.IgG is also involved in complement fixationReference: Harper biochemistry 30th edition page no 681