What term describes the initial immune response when a foreign substance is encountered for the first time?

• A. Primary response
• B. Memory response
• C. Secondary response
• D. Tertiary response

Answer: (A)

When a foreign substance is encountered for the first time, the immune system mounts the primary response. Naive B and T cells are activated, they proliferate and differentiate into plasma cells that produce antibodies (often starting with IgM), and this takes time, so antibody levels rise slowly. Because this is the initial exposure, memory B and T cells aren’t present yet, making the response slower and less robust. From this first contact, memory cells are created, which set the stage for a faster, stronger response if the same antigen shows up again—this subsequent, more rapid reaction is the secondary response. The memory response describes what happens on later exposures, not the first one, and tertiary isn’t the standard term used for these sequential immune encounters.