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
Which immunoglobulin acts as receptor on B cell? 
Select and explain. 
A. Ig G, B. Ig A, C. Ig M, D. Ig D
C: Ig M
Answer: C: Ig M 
Explanation: Ig M REF: Jawetz's, Melnick, & Adelberg's Medical Microbiology, 24TH edition chapter 8. Immunology B-cell Receptor for Antigen: B cells express a form of IgM that is located on the cell surface. Cell surface IgM has the same antigen specificity as the secreted IgM antibody molecule. This is achieved by a differential RNA splicing mechanism. The -chain RNA transcript can include a sequence that encodes about 25 hydrophobic amino acids, which enables the IgM molecule to localize in the cell membrane as a trans membrane receptor. Later in development of the B cell, regulation of RNA processing allows expression of a membrane-bound form of IgD, again with the same antigen-binding specificity. Throughout this process, the same V region segment is being expressed with different C region segments.