Detection of activated T-cells

A method of determining whether exposure of a first population of T-cells to a second population of T-cells causes activation of said first population of T-cells, said method comprising

• labeling the cells of said first population of cells with a first fluorophore which is also capable of labelling said second population of cells,
• contacting said first population of cells with said second population of cells to form a mixture of cells,
• the cytoplasm of said first population of cells being labelled with a second fluorophore which, when complexed with calcium ions or exposed to increased pH, emits or excites at a wavelength different from the wavelength which it emits or excites when not exposed to increased pH or complexed with calcium ions, or having an altered quantum efficiency when so exposed or complexed,
• placing said mixture of cells in a dual laser flow cytometry system,
• illuminating said mixture of cells with a first laser capable of causing said first fluorophore to emit light of a predetermined wavelength to cause self-identification of said first population of cells, and
• illuminating said mixture of cells with a second laser capable of causing said second fluorophore to emit light of a predetermined weavelength as an indication of activation of said first population of cells.