Which antibody is typically the first to appear in a primary immune response?

• A. IgD
• B. IgM
• C. IgG
• D. IgA

Answer: (B)

In a primary immune response, the first antibody produced is IgM. Naive B cells, upon encountering antigen, quickly differentiate into plasma cells that secrete IgM as the initial antibody. Its pentameric form gives it high avidity, helping to trap and neutralize pathogens early even if each individual antibody has only moderate affinity. As the response progresses, B cells undergo class switch recombination with T cell help, shifting production to other isotypes like IgG, IgA, or IgE that often have higher affinity after somatic hypermutation. IgD mainly serves as a B cell receptor on naive B cells and isn’t the primary circulating antibody early in the response, and IgA is more associated with mucosal secretions and appears later. Thus IgM appears first.