Final answer:
The line of defense that includes active immunity is the third line of defense. Active immunity is acquired when the body's immune system produces its own antibodies in response to an infection or through vaccination.
Step-by-step explanation:
The line of defense that includes active immunity is the third line of defense. Active immunity is acquired when the body's immune system produces its own antibodies in response to an infection or through vaccination. This type of immunity is long-lasting and provides protection against specific pathogens in the future. An example of active immunity is when someone gets a vaccine, such as the flu shot, and their immune system responds by producing antibodies.