Firstly, we note the great intuitions of Zacharias Jansen and Antoni von Leeuwenhoek, who were able to see ‘cells’ by seeing them better through an instrument made up of several stacked lenses, the microscope. Secondly, we describe the discoveries of Paul Ehrlich, who by using dyes, was able to differentiate both the cells’ structure and function. The preliminary findings by Ehrlich led to subsequent important discoveries not only in hematology but also in immunology and immunopathology.