Microbiology Practice Exam

What characteristic distinguishes IgM from other immunoglobulins?

1. Monomer

2. Dimer

3. Pentamer

4. Trimer

The correct answer is: Pentamer

The defining characteristic of IgM is its structure as a pentamer. This means that IgM is composed of five monomer units linked together, creating a large, complex molecule. This pentameric structure allows IgM to effectively bind to multiple antigens simultaneously, which is essential for its role in the initial immune response. It is typically the first antibody produced during an infection and is crucial for agglutination and activating the complement system, enhancing its ability to neutralize pathogens. In contrast, other immunoglobulins like IgG and IgA can exist primarily as monomers or dimers. IgG is predominantly a monomer in its functional form, while IgA can be found as a monomer or a dimer, particularly in secretory forms such as those found in mucosal surfaces. The unique pentameric configuration of IgM is critical for its function and distinguishes it from these other isotypes. This structural characteristic contributes significantly to its ability to trigger immune responses effectively.