Staining and selective affinity
Ehrlich obtained his medical doctorate in 1878 through research on tissue staining and subsequently studied staining of blood cells and pathogens in Berlin. His interest in the differing affinities of chemicals for cells also informed his later immunological and drug research.
References: [1]
Serum standards and side chains
Ehrlich helped standardize the potency of diphtheria antitoxin sera and developed the side-chain theory to explain relationships between toxins, cells and antibodies. His shared 1908 Nobel Prize recognized work on immunity and preceded the announcement of Salvarsan.
Research with Sahachiro Hata
At Frankfurt, Ehrlich organized screening of many compounds for selective action against pathogens. After joining him, Hata tested arsenical compound 606 in infected animals. The drug subsequently became known as Salvarsan and entered the history of early chemotherapy against syphilis.
References: [1]