How is immunity defined?

Immunity is defined as the body's ability to recognize and defend itself against pathogens, such as bacteria and viruses, providing protection against diseases. This definition encompasses both innate immunity (the body's natural defenses) and acquired immunity (the immune response developed after exposure to specific pathogens). This protective function is crucial as it helps the body not only to fight off current infections but also to prevent future ones.

In contrast, the other options do not accurately capture the essence of immunity. The ability to recover quickly from illness relates more to resilience or the effectiveness of treatment rather than the immune system's protection mechanisms. The capability to adapt to environmental changes pertains to biological or ecological concepts rather than specifically to health or disease prevention. The absence of health complications does not indicate immunity; one can be healthy without being immune to certain diseases, as immunity specifically involves the active defense system against pathogens.