Physikalisch-Technische Bundesanstalt

They really can be counted individually:

Red and white blood cells, blood platelets and other, rare, cells. The number of different white blood cells, for example, furnishes valuable information about the state of the immune system. For the counting process, the cells pass one after another through a very narrow tube whose electrical conductivity is measured. The value of the cell is different from that of the ambient fluid so that each individual cell can be detected. A novel method evaluates the blood sample without chemical preparation by optical means and thus allows the different white blood cells to be precisely differentiated. Coloured laser beams which are absorbed and refracted by the blood cells in a different way allow scattering diagrams typical of different diseases to be obtained.