Which statement correctly describes the primary immune response?

• A. It is faster and relies on existing memory cells
• B. It is the rapid response upon re-exposure
• C. It is slower and involves clonal expansion and memory formation
• D. It does not involve lymphocytes

Answer: (C)

When the body first encounters an antigen, the initial immune response starts with naive B and T lymphocytes being activated. These cells proliferate extensively in a process called clonal expansion, producing many identical cells specific for that antigen. B cells then differentiate into plasma cells that secrete antibodies and into memory B cells, while T cells expand into effector cells. Because this activation and expansion come from a relatively small naive pool, it takes several days to weeks to reach peak antibody levels. The memory cells formed during this encounter are designed to respond much faster if the same antigen is encountered again, creating a quick and stronger secondary response. So the primary response is slower and involves clonal expansion and memory formation, and it does involve lymphocytes. The rapid response on re-exposure refers to the secondary response, not the primary one.