How does the structure of IgE differ from that of IgG?

• A. IgG has more antigen-binding sites than IgE.
• B. IgE has one more constant region than IgG.
• C. IgG has a secretory component and IgE does not.
• D. IgG has more light chains than IgE.

Answer: (B)

The main idea tested is how the heavy-chain constant region count differs between antibody isotypes. Each antibody is built from two identical heavy chains and two identical light chains, but the number of constant domains in the heavy chain varies by isotype. IgG has three constant regions in its heavy chain, whereas IgE has four. So IgE actually has one more constant region than IgG. That structural difference underlies why the statement about IgE having one more constant region is the best choice. The other options don’t fit because both IgG and IgE are generally bivalent (they have two antigen-binding sites), the secretory component is associated with secretory IgA and some IgM, not IgG, and light-chain count is the same for both.