What defines active immunity?

Active immunity is defined as the immunity developed from natural infection or exposure to antigens. This form of immunity occurs when an individual’s immune system is exposed to pathogens, and in response, it produces specific antibodies against those pathogens. This can happen through actual infection by a virus or bacterium, resulting in the body building a memory that allows for a more effective and quicker response if the same pathogen is encountered in the future.

The process of developing active immunity is important because it not only provides long-term protection but also enables the immune system to recognize and respond to pathogens effectively over time. This recognition is facilitated by memory cells that remain in the body even after the initial infection has resolved. This distinguishes active immunity from passive immunity, where antibodies are transferred from one organism to another, such as from mother to baby.

In summary, the development of active immunity through natural infection or exposure to antigens is crucial for long-lasting immunity to specific diseases, which is why this option is the correct choice.