Many patients suffering from eczema, neurodermatitis, pruritus and other allergic skin diseases, the skin often itchy, patients often can not help but scratch the skin with finger nails, the result is often the skin itch is not reduced, but itch more powerful, so that the affected skin ulcers, pus, but also increased pain. Itchy skin, why not to scratch? Why does it get itchier and itchier after scratching? This can be explained in the following three ways. One, when scratching the skin, the hard fingernails are like sharp spatula, the thin and tender epidermis is peeled off, the bright red dermis is exposed, the tiny capillaries of the dermis rupture and bleed, at this time not only the natural barrier function of the skin disappears, but also the serum protein oozing from the dermal capillaries, which becomes the medium for bacteria, fungi and other microorganisms, often leading to various purulent bacterial infections, making the skin These two nerve impulses reach the sensory nerve center of the cerebral cortex through the nerve conduction pathway, forming the sensation of itch and pain, so after scratching the skin hard, the affected skin is painful and itchy. Second, the shed epidermal cells, capillary wall cells, serum and other such autologous tissue proteins, combined with bacteria to form an antigenic substance. This auto-antigen is absorbed into the bloodstream through the capillaries of the dermis and stimulates the body to have an allergic reaction, resulting in a red rash and increased itching, a phenomenon medically known as “auto-sensitivity reaction”. The inflammatory reaction stimulates the tiny nerve fibers in the dermis and produces slow-frequency nerve impulses. The slow-frequency nerve impulses cause the sensory nerve bier in the cerebral cortex to produce itchiness, so the itchiness increases after scratching the skin vigorously. Thirdly, scratching the skin directly damages the tiny nerve fibers in the dermis, and inflammatory reactions occur, causing the nerve fibers to produce slow-frequency nerve impulses, which also results in the cortical sensory nerve centers to produce itchiness, therefore, when the skin itches, do not scratch hard. A reasonable way to stop itching is to lower the skin temperature to numb the itchiness of the cerebral cortex with skin cold sensation. Of course, allergic skin diseases such as eczema, neurodermatitis and pruritus require comprehensive treatment with a variety of internal and external medications for different intrinsic and extrinsic pathogenic factors.