How does the immune system remember past encounters with antigens? This review defines immunological memory as the faster, stronger response to re-exposure to the same antigen. Memory cells differ from naive cells in surface markers and functional responses. Murine memory cells are CD44 high and low in activation markers, while human memory cells are CD45RA−, CD45RO+. Memory cells secrete a range of T cell cytokines and can be polarized to specific secretion patterns. They require less stringent activation than naive cells, needing costimulation for optimal responses and suboptimum antigen concentrations. Memory cells persist without antigenic stimulation but can expand upon re-encounter with the same antigen. Competition from other antigens can reduce the population. Arising unclear pathways, memory cells have different recirculation and homing patterns. This review clarifies the key aspects of T cell memory.
Published in the Annual Review of Immunology, this review of T cell memory aligns perfectly with the journal's focus on immunological processes. Its synthesis of current knowledge on memory cell characteristics and activation requirements makes it a valuable resource for immunologists.