Proteins with different isoelectric points in spermatozoon and cells carry different charges under the same pH and thus they will selectively bind to different stains. The amino liberated from acidophilic proteins carry positive charges and bind acid stain (Eosin) to become red. The carboxyl liberated from basophilic protein carry negative charges and bind alkaline stain (Methylene blue) to stain blue. Neutrophil proteins, whose positive charges from the liberated amino are equivalent to the negative charges of liberated carboxyl, bind simultaneously both the acid stain and alkaline stain, and appear mauve. However, as the liberated charges are equal, the stain is weak.