What does IgM describe in immune responses?

• A. Primary response occurring early in incubation or exposure period
• B. Secondary response occurring during convalescence
• C. IgM is a lipid molecule
• D. IgM is identical to IgG

Answer: (A)

IgM describes the primary immune response that occurs early after initial antigen exposure. When a person first encounters an antigen, B cells produce IgM first, so it appears early in the incubation or exposure period before the immune system switches to producing other classes like IgG. IgM is a pentamer, which gives it high overall avidity and makes it effective at agglutinating pathogens and activating the complement system for rapid defense. As the response progresses, memory B cells generate higher-affinity IgG during the secondary response, so IgM is less dominant. The other statements don’t fit because IgM is not a lipid molecule and it is not identical to IgG; the secondary response is primarily driven by IgG.