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
[{'content': 'True about IgM \nSelect and explain. \nA. Fix Complement, B. Increased in primary response, C. Fab region is composed of variable region, D. Fc is antibody - binding option', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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', 'role': 'assistant'}]