challenge: What is the composition of the Fc region in IgM and IgE antibody isotypes?
  
  A. One heavy chain constant domain in each polypeptide chain
  B. Four heavy chain constant domains in each polypeptide chain
  C. Three heavy chain constant domains (CH domains 2–4) in each polypeptide chain
  D. Two identical protein fragments, derived from the second and third constant domains of the antibody's two heavy chains
  Answer: 