What describes the longevity of active immunity?

Active immunity is a form of immunity that is gained through the natural process of infection or through vaccination. When the immune system is exposed to an antigen, it generates a response where memory cells are created. These memory cells enable the body to remember and respond more effectively when encountering the same pathogen in the future.

The longevity of active immunity is characterized as life-long or long-lasting because, once established, it usually provides protection against the specific pathogen for many years, and often for a person's entire life. This is particularly evident with diseases like chickenpox and measles, where once an individual has been infected or vaccinated, the immunity generally persists due to the robust memory B and T cells formed during the initial response.

While some vaccines may require booster shots to maintain immunity, the essence of active immunity is its ability to provide a durable defense without the need for frequent re-exposure. This distinguishes it from passive immunity, which is temporary and provides immediate but short-lived protection. Hence, the correct option captures the enduring nature of active immunity effectively, making it a key concept in understanding how vaccinations and natural infections contribute to long-term immunity against diseases.