(ār´lik) Paul (1854–1915). German bacteriologist. He studied medicine and was early drawn to research on aniline dyes. He did vast work on the problems of serology and immunity , and is known preeminently for his discovery of salvarsan or “606,” an arsenical compound later called arsphenamine, which was a cure for syphilis and the first effective chemotherapeutic agent against a microbial disease. He differentiated the leukemias , classified the leukocytes , described polychromatophilia , and is generally regarded as the founder of hematology . In 1908 shared with Metchnikoff the Nobel prize for medicine or physiology for his work in immunology .