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
Earliest immunoglobulin to be synthesized by foetus in response to infection: September 2010 
Select and explain. 
A. IgA, B. IgE, C. IgM, D. IgG
C: IgM
Answer: C: IgM 
Explanation: Ans. C: IgM In germline cells, the gene segment encoding the p (mu) constant region of the heavy chain is positioned first among other constant region gene segments. For this reason, IgM is the first immunoglobulin expressed by mature B cells. It is also the first immunoglobulin expressed in the fetus (around 20 weeks) and also phylogenetically the earliest antibody to develop