The initial response to a foreign antigen is known as the primary response. How long does the primary response usually take to occur?

• A. 2-6 days
• B. 2-6 months
• C. 2-6 weeks
• D. 2-6 hours

Answer: (B)

After first exposure to a foreign antigen, the immune system goes through a lag phase while naive B cells are activated, proliferate, and differentiate into antibody-secreting plasma cells. Because this involves clonal expansion and class switching, antibodies don’t appear immediately. The primary response is typically detectable in about one to two weeks, with IgM appearing first and IgG following. Among common time frames, roughly two weeks to six weeks best matches the onset of the primary response. Shorter windows like 2-6 days or 2-6 hours are too fast for a primary response, and 2-6 months is longer than the initial response period.