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 immunoglobulin has no known function but is present on the surface of B lymphocytes and it may function as an antigen receptor? 
Select and explain. 
A. IgA, B. IgE, C. Ig D, D. IgM
C: Ig D
Answer: C: Ig D 
Explanation: IgD antibody consists of two light chains and two heavy chains. Its role is not known but can be found on the surface of lymphocytes where it may act as a surface receptor. Ref : Ananthanarayana textbook of Microbiology 9th edition Pgno : 98