Which antibody class is characterized as a pentamer?

The correct answer is IgM, which is characterized as a pentamer. This means that an IgM molecule is composed of five monomeric units of antibodies joined together. This structural configuration allows IgM to effectively bind to multiple antigens simultaneously, making it an excellent choice for initiating the immune response. Typically, IgM is the first antibody produced in response to an infection, playing a crucial role in the early stages of the immune response.

In contrast, the other antibody classes mentioned have distinct structures and functions. IgE is primarily associated with allergic responses and is a monomer. IgD, which is found on the surface of immature B cells, also exists as a monomer, and while it plays a role in B cell activation, it is not involved in direct immune responses to pathogens in the same way as IgM. IgG is the most abundant antibody in serum and is a monomer; it is crucial for long-term immunity and can neutralize toxins and viruses, but it does not have the pentameric structure that characterizes IgM.