Answer: D. It is useful only when an antigen invades a body for the second time.
Explanation: Adaptive immunity, also referred to as acquired immunity, comes into play when innate immunity is breached. Adaptive immunity isn't very helpful in protecting your body when specific bacteria, virus, or any other microorganism enter your body for the first time. Its components function by learning the pathogen and then developing immunity against it. **(The adaptive immune system develops antibodies against the invaded pathogen after the first entry)**. If the same or similar pathogen enters the bloodstream again, your body will have a full-blown defense ready to destroy the pathogen. The mechanism of adaptive immunity is essentially concerned with your body's capability to recognize and destroy substances which it interprets as foreign and harmful... PF.
