Two unrelated individuals receive the seasonal influenza vaccine. You would expect their antibodies to the vaccine immunogens to be of the same which?

• A. Allotype
• B. Autotype
• C. Heterotype
• D. Isotype

Answer: (D)

The key idea is antibody class, or isotype. The isotype of an antibody is determined by the heavy-chain constant region and defines its functional class (IgM, IgG, IgA, IgE, IgD). When the immune system responds to a protein vaccine like the influenza immunogens, it typically undergoes class switching from IgM to a dominant systemic isotype, which in most people is IgG. So two unrelated individuals vaccinated against influenza would primarily produce antibodies of the same isotype (IgG) against the vaccine antigens, providing a similar protective function.

Allotype refers to inherited variants in the constant region that differ between individuals, so that wouldn’t be the same across unrelated people. Idiotype/autotype describe the unique antigen-binding regions of the antibodies, which vary between individuals. Heterotype isn’t a standard term for this context.