Chemical substances from the group of phenols (polyhydroxyphenols) that occur in the barks of many trees, shrubs and bushes as well as in the skins of fruits and also in grapes, especially of red wine varieties. They ultimately serve as a defence for plants against herbivores or predators and also as protection against microorganisms. Tannins can inactivate a large number of viruses and therefore have an antitoxic effect. They also have a very strong tanning effect, which is why they have been used for thousands of years in the production of leather. For this reason, they are often colloquially referred to as tanning agents, although this term tends to have a negative or pejorative connotation towards tannins.