Which type of immunity is characterized by an immediate response due to prior exposure to a pathogen?

The type of immunity characterized by an immediate response due to prior exposure to a pathogen is adaptive immunity. This form of immunity is specifically tailored to respond to pathogens that the body has encountered before. When a person is exposed to a pathogen, the adaptive immune system creates memory cells that remember how to effectively target that pathogen. As a result, upon re-exposure, the immune response is swift and robust, often neutralizing the pathogen before it can lead to infection or illness.

Adaptive immunity is distinct from other forms of immunity in that it involves a specific response to a particular pathogen and can adapt and improve with subsequent exposures. This mechanism is crucial in developing immunity after infections or vaccinations, where the body learns to defend itself effectively against specific invaders.

In contrast, innate immunity is the body's first line of defense and provides a rapid but non-specific response, while passive immunity involves the transfer of antibodies from one individual to another and does not involve the activation of the host's immune system. Active immunity refers to the body's own immune response to a pathogen or vaccination but does not necessarily imply an immediate response since it can take time to develop after the initial exposure.