Which antibody isotype is produced first during a primary immune response?

• A. IgA
• B. IgM
• C. IgG
• D. IgE

Answer: (B)

During a primary immune response, naive B cells first respond by producing IgM antibodies. Naive B cells express IgM (and IgD) on their surfaces as their B-cell receptor. When they encounter antigen, these cells quickly secrete IgM, providing rapid defense through its pentameric form, which efficiently binds antigen and strongly activates the complement system. This early response is followed by class switch recombination, guided by helper T-cell signals, which can change the antibody production to other isotypes such as IgG, IgA, or IgE depending on the cytokine environment and infection site. So the initial antibody produced in a primary response is IgM, with other isotypes coming in later as the immune response matures.