What is a characteristic of adaptive immunity?

Adaptive immunity is characterized by its ability to develop a targeted response to specific pathogens, which typically happens after the initial, non-specific response of the innate immune system. When the innate immune system, which includes physical barriers like skin and mucous membranes as well as cellular responses from phagocytes and natural killer cells, is not sufficient to eliminate the pathogen, the adaptive immune system is activated. This system is highly specific and develops memory, allowing for a faster and more efficient response upon subsequent encounters with the same pathogen.

The specific nature of adaptive immunity arises from the actions of T cells and B cells, which recognize and respond to antigens. This characteristic is crucial for vaccine effectiveness, as vaccines function by stimulating the adaptive immune response to prepare the body for future exposures to pathogens.

While options that include characteristics such as being non-specific or referring to barriers are associated with the innate immune response, adaptive immunity fundamentally relies on its ability to generate a tailored defensive action against specific threats. This highlights the sequential nature of the immune response, where adaptive immunity is often called into play following the innate response's shortcomings.