Editing Paul Ehrlich (section)

===Hematological staining===
In the early 1870s, Ehrlich's cousin [[Karl Weigert]] was the first person to stain bacteria with dyes and to introduce aniline pigments for histological studies and bacterial diagnostics. During his studies in Strassburg under the anatomist [[Heinrich Wilhelm Waldeyer]], Ehrlich continued the research started by his cousin in dyes and staining tissues for microscopic study. He spent his eighth university semester in Freiburg im Breisgau investigating primarily the red dye dahlia (monophenylrosanilin), giving rise to his first publication.<ref>{{cite journal |title=Beiträge zur Kenntniss der Anilinfärbungen und ihre Verwendung in der mikroskopischen Technik |journal=Archiv für Mikroskopische Anatomie |volume=13 |year=1877 |pages=263–277 |url=https://www.biodiversitylibrary.org/item/49005#page/287/mode/1up |doi=10.1007/BF02933937 |last1=Ehrlich |first1=P. |s2cid=84802859 }}</ref>

In 1878 he followed his dissertation supervisor [[Julius Friedrich Cohnheim]] to Leipzig, and obtained a doctorate with a dissertation entitled "Contributions to the Theory and Practice of Histological Staining" (''Beiträge zur Theorie und Praxis der histologischen Färbung'').

[[File:SMCpolyhydroxysmall.jpg|thumb|Photo of cultured mast cells at 100× stained with [[Tolonium chloride|Tol Blue]]]]

One of the most outstanding results of his dissertation investigations was the discovery of a new cell type. Ehrlich discovered in the [[protoplasm]] of supposed [[plasma cells]] a granulate which could be made visible with the help of an alkaline dye. He thought this granulate was a sign of good nourishment, and accordingly named these cells [[mast cell]]s, (from the German word for an animal-fattening feed, ''Mast''). This focus on chemistry was unusual for a medical dissertation. In it, Ehrlich presented the entire spectrum of known staining techniques and the chemistry of the pigments employed.
While he was at the Charité, Ehrlich elaborated upon the differentiation of white blood cells according to their different granules. A precondition was a dry specimen technique, which he also developed. A drop of blood placed between two glass slides and heated over a Bunsen burner fixed the blood cells while still allowing them to be stained. Ehrlich used both alkaline and acid dyes, and also created new "neutral" dyes. For the first time this made it possible to differentiate the [[lymphocyte]]s among the [[leucocyte]]s (white blood cells). By studying their granulation he could distinguish between nongranular lymphocytes, mono- and poly-nuclear leucocytes, [[eosinophil granulocyte]]s and mast cells.

Starting in 1880, Ehrlich also studied [[red blood cell]]s. He demonstrated the existence of nucleated red blood cells, which he [[Erythropoiesis|subdivided]] into normoblasts, megaloblasts, microblasts and poikiloblasts; he had discovered the precursors of erythrocytes. Ehrlich thus also laid the basis for the analysis of [[anemia]]s, after he had created the basis for systematising [[leukemia]]s with his investigation of white blood cells.

His duties at the Charité included analysing patients' blood and urine specimens. In 1881 he published a new urine test which could be used to distinguish different types of typhoid from simple cases of diarrhea. The intensity of staining made possible a disease prognosis. The pigment solution he used became known as [[Ehrlich's reagent]].
Ehrlich's great achievement, but also a source of problems during his further career, was that he had initiated a new field of study interrelating chemistry, biology and medicine. Much of his work was rejected by the medical profession, which lacked the requisite chemical knowledge. It also meant that there was no suitable professorship in sight for Ehrlich.