Final answer:
Adaptive immunity is an acquired defense mechanism in which the body develops a specific immune response to a pathogen either after natural infection or through vaccination, leading to the formation of immunological memory.
Step-by-step explanation:
Adaptive Immunity : A form of acquired immunity that develops in an individual in response to an immunogen (antigen) is known as adaptive immunity. This type of immunity is characterized by the ability to form memory lymphocytes (B or T cells) after exposure to a specific pathogen or vaccination. It consists of two types: naturally acquired active immunity and artificially acquired active immunity. Naturally acquired active immunity is obtained through exposure to the live pathogen and developing an immune response. For example, someone recovering from chickenpox would usually have lifelong immunity against future chickenpox infections. Artificially acquired active immunity comes from immunizations, where a vaccine containing a killed or weakened form of the pathogen, or parts of it, is administered to stimulate the immune system without causing illness. Vaccination is key to this process, enabling the body to resist the pathogen by having previously developed a specific response highly effective upon re-exposure to the same pathogen. Thanks to vaccines, diseases like smallpox have been eradicated, showcasing the importance of adaptive immunity in public health.