Final answer:
Active immunity is the ability of the adaptive immune system to resist a specific pathogen by forming an immunological memory. It can occur naturally through infection or artificially through vaccination. Memory cells provide long-term protection against future infections from the same pathogen.
Step-by-step explanation:
Active immunity is the ability of the adaptive immune system to resist a specific pathogen because it has formed an immunological memory of the pathogen. Active immunity can come about naturally or artificially. It occurs when the immune system responds to an infection or vaccination and produces memory cells, which provide long-term protection against future infections from the same pathogen. For example, when a person receives a vaccination, such as a flu shot, it triggers an immune response and stimulates the production of memory cells, resulting in active immunity. This means that if the person is exposed to the flu virus in the future, their immune system will recognize it and mount a quick and effective response.