Which immunoglobulin rises in acute infections at 3-6 weeks?

• A. IgM
• B. IgG
• C. IgA
• D. IgE

Answer: (A)

The key idea is how the antibody response unfolds in an acute infection. When the body first encounters a pathogen, it mounts an early antibody response with IgM. IgM is produced quickly and rises during the acute phase, helping to neutralize pathogens and activate complement thanks to its pentameric structure, which enables strong agglutination and rapid initial defense. As the response matures, B cells undergo class-switching and produce IgG, which becomes the dominant antibody later and provides long-term, high-affinity protection. IgA specializes in mucosal immunity, while IgE is mainly involved in allergy and defense against certain parasites. So, IgM is the immunoglobulin that rises prominently in the early acute phase of infection.