The ability of the immune system to mount a secondary immune response, i.e. to respond faster and more effectively to subsequent exposures to an antigen following an initial, or primary, immune response to the same antigen (e.g. as a result of an infection or immunization). It depends on the formation of a small and persistent population of memory cells, induced by the primary immune response. Memory T cells appear typically only five days following initial immunization, whereas memory B cells may take about a month to reach maximum levels. Populations of such cells persist for the lifetime of the individual and exhibit enhanced affinity for the antigen, especially with repeated exposure to the original antigen. When memory cells encounter their specific antigen, they rapidly divide to produce a clone of effector cells capable of dealing with the target. Memory B cells undergo changes in their immunoglobulin genes that enable them to produce antibodies with greater affinity for the antigen, resulting in its efficient disposal.