What are memory T cells?

A memory T cell is an "experienced" T cell. While normal infection/cancer fighting T cells will mount a defense within the body by replicating themselves to fight off the infection. A memory T cell is able to mount a defense and replicate itself faster because it has encountered a particular type of invader before.

T cells gain memory by fighting off infections or by encountering injected vaccinations in the blood stream. (vaccinations contain dead infections which cannot infect the body, but the T cells still latch onto them and remove them and thus learn how to fight off that infection if it were alive). Memory T cells can also be developed in young children during breast feeding as they will learn from the strains passed over from the mother which have already been killed off by her immune system.

T cells are why we are able to get sick and recover within a few days. Memory T cells are why we are able to either avoid the same strain of infection or at least reduce its recovery time if we encounter it again.