Final answer:
The part of the immune system that needs to be activated and has the ability to 'learn' is the adaptive immune system, which includes B and T cells that provide specific, long-lasting immunity and memory against pathogens.
Step-by-step explanation:
The part of the immune system that requires activation and is characterized by its ability to 'learn' and develop immunological memory is the adaptive immune system. Unlike the innate immune system, which provides quick but non-specific responses, the adaptive immune response is highly specific to an invading pathogen and takes longer to put into effect. However, once it is activated, the adaptive immune system, through B and T cells, not only targets the pathogen very effectively but also retains a memory of it. This memory ensures that subsequent encounters with the same pathogen will trigger a faster and stronger response, usually preventing symptoms of illness from developing.