Microbiology Practice Exam

What immunoglobulin is known as the first antibody to respond to an infection?

1. IgG

2. IgA

3. IgE

4. IgM

The correct answer is: IgM

The first antibody to respond to an infection is IgM. This immunoglobulin is typically produced and secreted by B cells in the early stages of an immune response. When an organism encounters an antigen, such as a pathogen, the immune system activates and begins to generate IgM antibodies. They are primarily found in the bloodstream and are effective in forming complexes with antigens, facilitating their removal from the body. IgM is structurally unique as it exists as a pentamer, which means that five monomer units are linked together. This structure allows it to effectively bind to multiple antigens at once, enhancing its capacity to neutralize pathogens. While IgG is the most abundant antibody in the serum and plays a critical role in later stages of the immune response, it is not the first to respond. Similarly, IgA is predominantly found in mucosal areas and secretions, such as saliva and tears, while IgE is primarily involved in allergic responses and protection against parasitic infections. Therefore, IgM's role as the first responder in the antibody-mediated immune response is crucial for the initial defense against infections.