As early as 1948, Tsai SS, a Japanese scholar, found that yam mucilage is composed of mannan protein molecular complex, that is, mainly the complex of sugar and protein. Later, the study found that yam mucilage also contains a small amount of allantoin, amino acids, lipids, diosgenin, catecholamine, dioscorene hydrochloride, etc. < / P > < p > it is the presence of proteins and sugars that makes it sticky. The viscosity of this mucus is affected by many factors such as concentration, ion concentration, acidity and temperature. The viscosity increases with the increase of concentration and decreases with the increase of ion concentration. The lower the temperature is, the higher the viscosity will be. < p > < p > itching is a kind of unpleasant feeling. It can cause people to scratch the desire or reflex, which belongs to a kind of body protection response. Itching substances can be divided into two categories: one is histamine. The receptors for these substances are mainly histamine receptors, including substances that stimulate mast cells to produce histamine and histamine, which is the source of the itching of mosquito bites. It can be simply thought that when there are red, swollen and epileptic marks, it is due to the itching caused by histamine. < / P > < p > the other is non histamine itching substances, such as chloroquine, short peptide sligrl, bromelain, chenopodin, etc. their main receptors are Mrgprs family protein (MAS related G protein coupled receptor) and PAR-2 receptor. Pruritus diseases such as chronic pruritus and contact dermatitis are closely related to Mrgprs. < / P > < p > when some people peel fresh yam, any place stained with mucus will cause severe pruritus and form contact dermatitis. This kind of itch is different from that of mosquito bite. The former does not immediately cause local skin redness, swelling, blisters or rashes, and the range of itching is larger. What is this itchy non histamine? Many people say that the itching of yam mucus is caused by saponins or saponins, but there is no literature report that the above substances are the main culprit of itching of yam. < / P > < p > in order to understand this problem, researchers extracted a substance from yam called allantoin, which belongs to hydantoin derivative, widely exists in animals and plants, and is also the metabolite of purine. Its content in yam is very small, only about 3.567 mg of allantoin per gram of yam. < / P > < p > next, the researchers did the following experiment with allantoin. They shaved the hair off the neck and back of mice and divided them into two groups. They injected allantoin into the neck of mice in the experimental group, and injected normal saline into the neck of mice in the control group. < / P > < p > the researchers further obtained the neurons of mouse dorsal root ganglion (DRG), and examined the cellular activity of itching substances by calcium imaging technology. The results showed that about 8% of DRG neurons responded to allantoin. Similarly, electrophysiological experiments showed that the action potential of diameter neurons could be directly induced by allantoin stimulation. This suggests that allantoin may be an itching substance in yam. < / P > < p > when dealing with yam, you can peel yam with gloves, or put yam in boiling water for two minutes, and then peel it. < / P > < p > in case the fresh yam mucus gets on the skin, you can blow it with a hair dryer or bake a fire next to the stove. The allantoin will decompose when heated, and it will not itch.