The acquired immune system only develops an immune response after an antigen has entered the body.

• A. True
• B. False
• C. It depends
• D. Not sure

Answer: (A)

The adaptive immune system activates only when it encounters a specific antigen. Naive B and T lymphocytes circulate until an antigen binds to their receptors or is presented by an antigen-presenting cell. This recognition triggers activation, clonal expansion, and differentiation into antibody-secreting cells and effector T cells, with memory cells formed for faster, stronger responses upon re-exposure. Vaccination uses harmless forms of antigen to train this system before real infection. Innate immunity, by contrast, provides non-specific defenses without requiring prior exposure. So the statement is true: the acquired immune response develops after an antigen has entered the body.