Fundamental to the concept of adaptive immunity is the ability of lymphocytes to recognize and respond to specific foreign Ags. B and T cells that are specific for peptides of an invading pathogen will proliferate and produce Abs, cytokines, and effector molecules and, upon resolution of the infection, form a long-lived memory response to protect against reinfection. However, for many decades, it was not possible for immunologists to track cells specific to a particular Ag over an extended time period. And although we have now delineated molecular markers of the many B and T cell subsets that occupy distinct spatial and functional niches, much of this progress arose from the physical ability to sort, classify, and track Ag-specific cells.