The ability of certain cells of the immune system to respond more robustly upon re-exposure to the same infection constitutes the basis for vaccination. These same cells that protect the host from repeat pathogen encounter make up the adaptive arm of the immune system. For decades, T and B cells have been known to provide immune memory after vaccination; however, several recent studies have demonstrated that natural killer (NK) cells also possess the adaptive immune feature of immunological memory.

The adaptive immune system is estimated to have arisen in lower vertebrates nearly 500 million years ago1. As a result of recombination-activating gene (RAG)-mediated recombination of genes encoding antigen recognition receptors, each individual T cell and B cell is endowed with the ability to specifically detect a specific antigen. Collectively, this army of lymphocyte clones can identify an infinite number of self and foreign antigens. Through well-characterized selection processes in the thymus and bone marrow, T and B cells, respectively, undergo an “education” to eliminate self-reactive clones, and generate an army of specialized lymphocytes that can recognize many pathogens and pathogen components.