Final answer:
Memory cells arise from activated B and T lymphocytes during the primary immune response. They remain dormant until re-exposure to the same pathogen, at which point they mount a rapid and effective immune response. This principle underlies the success of vaccinations in providing long-term immunity.
Step-by-step explanation:
Following a primary immune response, the cells that give rise to memory cells are specifically activated B and T lymphocytes. During a primary immune response, these cells encounter a pathogen for the first time. A subset of the activated B cells and T cells differentiates into memory cells. These memory B cells and memory T cells maintain the antigen specificity that was responsible for their initial activation.
Unlike effector cells, which respond immediately and then die off after dealing with the pathogen, memory cells do not participate in the immediate defense response. Instead, they persist in the body for years or even decades, allowing for a faster and more robust response upon subsequent exposures to the same pathogen. This leads to a quicker, more efficient secondary immune response that can often prevent the pathogen from causing disease symptoms, a principle which is utilized when we vaccinate against diseases.