Question

Explain the role of memory cells in long term immunity?

Answer 1

During first contact with the pathogen, immature T and B cells undergo clonal selection and increase in population of those cells, which can deal with the pathogen because of correspondence between its antigens and organization of their receptors. It takes much time, and thus primary specific immune response is delayed. Some of the mature T and B cells become memory cells, which preserve their antigenic specificity and can develop into mature T and B cells immediately, when needed. Thus, the second case of contact with the same pathogen leads to fast and efficient prevention of disease. In addition, memory cells are long-living, ensuring long-term immunity.
A good image to illustrate it is https://www.tcells.org/scientific/images/memorytcells.gif
You can find more info here https://www.ncbi.nlm.nih.gov/books/NBK27158/