
However, fundamental questions about the signals that are required to generate memory cells, and the mechanisms responsible for the survival of these cells in the apparent absence of antigen, remain unsolved. Much effort has been devoted to defining the phenotypic and functional characteristics of memory cells, and much has been learned about what these cells do. The classical definition of memory came from the realization that infection with a particular microbe or vaccination rendered an individual resistant to subsequent exposures to that microbe that is, they were immune to the specific pathogen.

Immunological memory refers to the ability of the immune system to respond more strongly to successive exposures to the same antigen.
