Read the following passage: "The human immune system is a complex network of cells, tissues, and organs that work together to protect the body from pathogens like bacteria, viruses, fungi, and parasites. It is broadly categorized into two main branches: the innate immune system and the adaptive (or acquired) immune system. The innate immune system provides immediate, non-specific defense against invaders. It includes physical barriers like skin, chemical barriers like stomach acid, and various immune cells such as phagocytes that engulf and destroy foreign particles. The adaptive immune system, in contrast, is highly specific and has a 'memory,' meaning it can recognize and respond more effectively to pathogens it has encountered before. This memory is crucial for long-term immunity, for example, after vaccination or recovery from an infection. Both systems are vital for maintaining health, with the innate system providing the first line of defense and the adaptive system offering targeted, long-lasting protection." Which of the following characteristics is unique to the adaptive immune system, as described in the passage?

1. AProviding immediate, non-specific defense
2. BEngulfing and destroying foreign particles
3. CActing as a physical barrier like skin
4. DRecognizing and responding to previously encountered pathogens