During a primary immune response, which antibody is produced first?

• A. IgG
• B. IgA
• C. IgE
• D. IgM

Answer: (D)

In a primary immune response, the immediate antibody produced is IgM. This happens because naive B cells first respond to antigen with their B cell receptor, which is membrane-bound IgM. Upon activation, these B cells differentiate into plasma cells that secrete IgM as the initial antibody. IgM is a pentamer, giving it high avidity and making it very effective at rapidly binding pathogens and activating the classical complement pathway to help contain the infection while other B cells undergo class switching. Over time, helper T-cell signals promote class switching to other isotypes, so later responses often yield IgG, IgA, or IgE depending on the tissue and type of pathogen. IgG, IgA, and IgE are produced later after this initial IgM response.