This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Microbiology
Which part is responsible for the class of immunoglobulin? 
Select and explain. 
A. Heavy chain, B. Light chain, C. Both, D. None
A: Heavy chain
Answer: A: Heavy chain 
Explanation: Ans. is 'a' i.e. Heavy chain * There are five immunoglobulin classes (isotypes) of antibody molecules found in serum: IgG, IgM, IgA, IgE and IgD. They are distinguished by the type of heavy chain they contain.* IgG molecules possess heavy chains known as g-chains; IgMs have m-chains; IgAs have a-chains; IgEs have e-chains; and IgDs have d-chains. The variation in heavy chain polypeptides allows each immunoglobulin class to function in a different type of immune response or during a different stage of the body's defense.