Editing Paul Ehrlich (section)

====First work on immunity====
Ehrlich had started his first experiments on [[immunization|immunisation]] already in his private laboratory. He accustomed mice to the poisons [[ricin]] and [[abrin]]. After feeding them with small but increasing dosages of ricin he ascertained that they had become "ricin-proof". Ehrlich interpreted this as immunisation and observed that it was abruptly initiated after a few days and was still in existence after several months, but mice immunised against ricin were just as sensitive to abrin as untreated animals.

This was followed by investigations on the "inheritance" of acquired immunity. It was already known that in some cases after a [[smallpox]] or syphilis infection, specific immunity was transmitted from the parents to their offspring. Ehrlich rejected inheritance in the genetic sense because the offspring of a male mouse immunised against abrin and an untreated female mouse were not immune to abrin. He concluded that the foetus was supplied with [[antibodies]] via the pulmonary circulation of the mother. This idea was supported by the fact that this "inherited immunity" decreased after a few months. In another experiment he exchanged the offspring of treated and untreated female mice. The mice which were nursed by the treated females were protected from the poison, providing the proof that antibodies can also be conveyed in milk.

Ehrlich also researched [[autoimmunity]], but he specifically rejected the possibility that an organism's immune system could attack the organism's own tissue calling it "horror autotoxicus". It was Ehrlich's student, [[Ernest Witebsky]], who demonstrated that autoimmunity could cause disease in humans.<ref>{{cite journal|last1=Witebsky|first1=Ernest|title=Chronic Thyroiditis and Autoimmunization|journal=Journal of the American Medical Association|date=27 July 1957|volume=164|issue=13|pages=1439–47|doi=10.1001/jama.1957.02980130015004|pmid=13448890}}</ref><ref>{{cite journal|title=Autoimmunity versus horror autotoxicus: The struggle for recognition|last1=Silverstein|first1=Arthur M.|journal=Nature Immunology|date=1 April 2001|volume=2|issue=4|pages=279–281|doi=10.1038/86280|pmid=11276193|s2cid=10275131}}</ref> Ehrlich was the first to propose that regulatory mechanisms existed to protect an organism from autoimmunity, saying in 1906 that "the organism possesses certain contrivances by means of which the immunity reaction, so easily produced by all kinds of cells, is prevented from acting against the organism's own elements".<ref>{{cite journal|doi=10.1146/annurev-cancerbio-030419-033428|doi-access=free|title=Regulatory T Cells in Cancer|year=2020|last1=Plitas|first1=George|last2=Rudensky|first2=Alexander Y.|journal=Annual Review of Cancer Biology|volume=4|pages=459–477}}</ref>