Which antibody is typically the first produced in the primary immune response?

• A. IgG
• B. IgA
• C. IgM
• D. IgD

Answer: (C)

The antibody produced first during a primary immune response is IgM. This happens because naive B cells initially respond to the antigen by making IgM as their default secreted isotype before class switching occurs. IgM is produced as pentamers, which gives high overall avidity and is especially effective at early pathogen neutralization and complement activation, helping to control infection before more specialized antibodies come into play. As the response matures, signals from helper T cells and cytokines drive class-switch recombination, so B cells start producing other isotypes like IgG, IgA, or IgE depending on the infection site and needs. IgG often becomes dominant later and provides memory, IgA is key at mucosal surfaces, while IgD mainly serves as a membrane-bound B cell receptor on naive B cells and is not the primary secreted antibody in the early response.