The early history of the eosinophil.

In 1879 Paul Ehrlich published his technique for staining blood films and his method for differential blood cell counting using coal tar dyes and mentions the eosinophil for the first time. Eosin is a bright red synthetic dye produced by the action of bromine on fluorescein and stains basic proteins due to its acidic nature. It was discovered in 1874 by Heinrich Caro, Director of the German chemical company Badische Anilin- und Soda-Fabrik. Ehrlich introduced the term 'eosinophil' to describe cells with granules (which he called alpha-granules) having an affinity for eosin and other acid dyes. He also observed black-staining, indulinophilic, beta-granules in bone marrow-derived eosinophils, which were probably immature crystalloid granules in eosinophil myelocytes. Ehrlich described the features of the alpha-granule and the cell's distribution in various species and tissues. He speculated correctly that the alpha-granule contents were secretory products and described several causes of eosinophilia including asthma, various skin diseases, helminths and reactions to medications. However, the cell was almost certainly observed by others before Ehrlich. In 1846 Thomas Wharton Jones (1808-1891) described 'granule blood cells' in the lamprey, frog, fowl, horse, elephant and man. He 'borrowed' the term granule cell from Julius Vogel (1814-1880) who had observed similar cells in inflammatory exudates. Vogel in turn was aware of the work of the Gottlieb (Théophile) Gluge (1812-1898) who used the term 'compound inflammatory globules' to describe cells in pus and serum. Almost 20 years before Ehrlich developed his staining methods, Max Johann Sigismund Schultze (1825-1874) performed functional experiments on coarse granular cells using a warm stage microscopic technique and showed they had amoeboid movement and phagocytic abilities. Although these early investigators recognised distinct granular cells Ehrlich's use of stains was a landmark contribution, which heralded modern studies on eosinophils and other blood leucocytes.