The secondary immune response is faster and much stronger than the primary response because
- A. the antigen is weaker the second time
- B. the skin has become impermeable to that pathogen
- C. memory cells produced in the first exposure are already present in large numbers
- D. antibodies from the first exposure are still circulating in the same amount
Explanation
A first exposure leaves behind long lived memory B and T cells specific to that antigen, so a second exposure finds a large clone ready to divide and antibody rises within a day or two instead of a week. Antibody from the first response falls to a low level over time, so persistence of antibody is not the main explanation. Vaccination and booster doses work by deliberately creating this memory.