Which immunoglobulin is first produced during an initial immune response?

• A. IgG
• B. IgA
• C. IgM
• D. IgE

Answer: (C)

In a first exposure to an antigen, the immune system starts by making IgM. Naive B cells that recognize the antigen become plasma cells and secrete IgM as the initial antibody, which appears early in the response. IgM has a pentameric structure, giving it high avidity and the ability to effectively agglutinate pathogens and activate the classical complement pathway, helping to control infection quickly before other defenses are mobilized.

As the response continues, signals from helper T cells drive class switching, so the body starts producing other antibody types such as IgG, IgA, or IgE, depending on the context. IgG often becomes the dominant isotype in subsequent encounters due to affinity maturation and memory B cells. IgA serves mucosal surfaces, while IgE is involved in allergic reactions and defense against parasites. That makes IgM the first immunoglobulin produced during an initial immune response.