Interpreting Primary and Secondary Immune Responses

Steps:

• Identify the primary response: initial lag (days 1-7), rise peaking around day 10-14, then decline.
• Locate the secondary response: sharper rise after re-exposure, starting around day 12-14 with higher peak.
• Note the timing: secondary activation aligns with memory cell involvement post-primary peak.
• Rule out options by matching graph phases to immune events.

Why B is correct:

• Memory cells, formed during primary response, enable rapid secondary response; activation on day 12 matches the start of accelerated antibody production per immunological memory definition.

Why the others are wrong:

• A: Active immunity develops within 7-10 days of primary exposure, not 25.
• C: Second exposure precedes secondary rise, occurring before day 12, not on day 25.
• D: T-helper cells persist longer than 20 days, aiding both responses without fixed duration.

Final answer: B