What defines acquired immunity?

Acquired immunity is primarily defined by its ability to provide immunological memory, which distinguishes it from innate immunity. This aspect allows the immune system to recognize and respond more effectively to pathogens that it has encountered previously. When an individual is exposed to a specific antigen through infection or vaccination, the immune response activates the formation of memory cells—these cells remain in the body long-term and enable a quicker and more robust response upon subsequent exposures to the same pathogen.

This characteristic is essential for long-lasting protection against specific diseases, ensuring that the immune system can remember past invaders and respond with heightened effectiveness. Vaccination takes advantage of this process by introducing a harmless component of a pathogen, prompting the body to create memory cells without causing disease.

In contrast, other options do not encompass the full definition of acquired immunity. For example, while vaccines and natural infections contribute to acquired immunity, limiting the definition to either of these sources alone does not fully capture the concept. Additionally, immediate responses without memory are indicative of innate immunity, which acts as the body's first line of defense and does not involve the formation of memory cells.