Immunologic memory is the characteristic component of the adaptive immune system. It is what confers lifelong protection from infections we have already been exposed to once.  Immunologic memory depends on the memory B cells and memory T cells that have a rapid and robust response after being re-exposed to the antigen that they are specific for. These memory cells develop during the first exposure to an antigen. When naïve B cells and T cells become activated upon antigen exposure, they differentiate into effector cells. However, a subset of these cells differentiates into memory cells. These memory cells persistent after the infection, unlike the effector cells which undergo apoptosis. Memory T cells become localized in lymphoid tissue and mucosal sites and memory B cells are found in circulation. As we progress through life our memory cells outnumber our naïve cells due to the myriad antigen exposures we undergo. Memory T cells, specifically, become the majority of T cells within adults. 

Immunologic memory coordinates the more rapid and vigorous response to the secondary exposure. The secondary exposure produces a much greater amount of antibody in a short amount of time. Memory B cells and memory T cells both have a faster activation in response to a secondary exposure. The memory T cells express a larger amount of MHC class II molecules on their cell surface which allows for more opportunities for binding to antigen. Memory B cells do not require the B7-CD28 co-stimulation which lessens the time to activation and differentiation. Moreover, these memory B cells have already undergone isotype switching and affinity maturation. Consequently, memory B cells become effector cells upon re-exposure and provide an immune response specific for that antigen.

Vaccination functions on the strength of immunologic memory. Vaccines introduce and antigen that a person has never been exposed to and this exposure generates a primary immune response and the development of memory T and memory B cells. This way, the next time a person is exposed to this antigen, they will have strong immune response that will keep them safe from infection.

References:

-Resource 1

-https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-immune-system-42/adaptive-immune-response-234/immunological-memory-878-12128/