Flow cytometry is a method for counting, sorting and isolating particles (e.g. living cells) which, after being specifically marked with a fluorescent dye, are individually moved past an optical detector system in laminar flow and then counted. By deriving parameters from this counting operation, it is possible to use this method to analyze many characteristics of the particles, whereby the property to be determined can be selected by the specific marking. One promising, current application is dye marking and isolation of antigenspecific cells within the framework of developing treatment methods for e.g. viral infections or cancer.