How did Paul Ehrlich's work with methylene blue contribute to the idea of chemotherapy?

Paul Ehrlich’s early experiments with methylene blue were foundational to the development of chemotherapy. As a medical student, Ehrlich utilized chemical dyes to stain cells for microscopic examination. He observed that these dyes exhibited a selective affinity for specific types of cells, suggesting that chemical reactions were the basis of cellular processes.1

This observation led Ehrlich to hypothesize that if a chemical could selectively stain a cell, it might also be used to selectively target and destroy disease-causing organisms without harming the host. He further explored the therapeutic potential of methylene blue in treating nervous disorders. These insights evolved into his concept of the "magic bullet"—a drug designed to specifically target pathogens. By recognizing that chemical agents could be used to heal diseased cells, Ehrlich moved beyond traditional serum therapy to pioneer the field of chemotherapy, eventually leading to the successful development of Salvarsan for the treatment of syphilis.123