Just 110 years ago in 1900, Paul Ehrlich, one of the founding fathers of modern immunology, gave the Croonian Lecture to the Royal Society in London "On Immunity with Special Reference to Cell Life". In this lecture he explained his "side-chain theory" proposing for the first time a receptor-ligand interaction as the basis for the immune reaction against bacterial toxins. This theory fell in discredit when it was found that our immune system does not only react against toxins but against a diverse universe of different molecular structures that are foreign to our body... 
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