Which immunoglobulin is most accurately described as involved in the early stages of immunity?

• A. IgA
• B. IgG
• C. IgM
• D. IgE

Answer: (C)

In a primary immune response, the first antibody produced is IgM. This early production is due to naive B cells switching on to secrete IgM before other isotypes. IgM’s pentameric structure, linked by the J chain, gives it high overall avidity and makes it especially effective at cross-linking polyvalent antigens and triggering the classical complement pathway, which helps clear pathogens quickly. Because of these features, IgM is the hallmark of the early stage of immunity. In contrast, IgG takes over in later, secondary responses and provides longer-term protection; IgA acts mainly at mucosal surfaces; and IgE is involved in allergic responses and defense against certain parasites.