To determine the function of a particular cell type, it is necessary either to have a large number of similar (ideally identical) cells or to use extremely sensitive methods to detect the activity of a single cell. Lymphocytes present special difficulties, because they have very precise antigen (Ag) recognition requirements, and, under physiological conditions, they will only be activated if they are exposed to their particular Ag. Polyclonal mitogens, such as phytohemagglutinin (PHA) or anti-CD3, will activate most T-cells, but may not elicit a truly physiological response in terms of cytokine production, and so on. Moreover, the biological readout (release of cytokines into culture supernatant) will represent the net balance of the integrated response of all the activated cells, minus any consumption of cytokines by the cultured cells.