Paul Ehrlich (1854-1915), German immunologist. Ehrlich worked with aniline dyes, developing new methods of staining white blood cells. He discovered a new type of white blood cell, called mast cells. Ehrlich also studied diseases, and helped to develop the diphtheria anti-toxin in 1892. He investigated synthetic chemicals that selectively destroy pathogenic organisms in the body. He discovered that the dye Trypan Red was effective against the parasites that cause trypanosomiasis (sleeping sickness) and used an arsenic compound called 'salvarsan' against syphilis. This was the beginning of chemotherapy. Ehrlich shared a Nobel Prize for medicine in 1908.