Which antibody is the first responder and often forms a pentamer?

• A. IgG
• B. IgA
• C. IgD
• D. IgM

Answer: (D)

When a new antigen is encountered, the immune system first produces IgM. This antibody is typically secreted as a pentamer, a structure formed by five IgM units linked together with a J chain. That pentamer arrangement gives IgM many binding sites, allowing it to clump multiple antigens together (agglutination) and to activate the classical complement pathway effectively, providing a rapid, early defense. While IgM is the initial responder during the primary response, other antibody types such as IgG, IgA, and IgD come into play later or in different contexts, with IgG being the more refined, high-affinity antibody produced after class switching and during secondary exposure, IgA functioning mainly at mucosal surfaces (often as dimers), and IgD acting primarily as a B-cell receptor. Therefore, the antibody that is the first responder and often forms a pentamer is IgM.