Which immunoglobulin is the first antibody produced in a primary immune response?

• A. IgG
• B. IgM
• C. IgA
• D. IgE

Answer: (B)

In a primary immune response, the first antibody produced is IgM. Naive B cells carry surface IgM, and when they encounter the antigen with help from T cells, they rapidly differentiate into plasma cells that secrete soluble IgM. IgM appears first because it’s the default isotype before any class-switching occurs. Its pentameric form gives high avidity and strong early neutralization and complement activation, providing initial protection while the more specialized isotypes are being prepared. Over time, signals from the immune environment drive class-switch recombination to other isotypes like IgG, IgA, or IgE, depending on the context. IgG becomes more prominent later in the response, IgA protects mucosal surfaces, and IgE is involved in certain allergic and anti-parasitic responses.