Itchy red spots on the knees may be caused by insect bite reaction, common type psoriasis, contact dermatitis and other diseases. 1. Insect bite reaction: this disease is related to insect bites, commonly found in mosquitoes, fleas, lice, etc. The allergen is the secretion injected into insect bites, and the main symptoms are redness, erythema, blisters, itching and other manifestations at the place of bites, so itchy knees with long red dots may be an insect bite reaction. 2. Ordinary-type psoriasis: the cause of this disease is not clear, but research has shown that it may be related to heredity, sunburn, trauma, viral infections, endocrine disorders, smoking and other factors, the typical symptom is the presence of dotted or mapped plaques on the skin, accompanied by itching, scaling, can be distributed throughout the body, especially elbows and knees, which may be manifested as itchy knees with long red spots. 3. Contact dermatitis: the common exogenous allergens of this disease are clothing materials, pollen, animal dander, plasters, etc. If the knees come into contact with allergens, they will be itchy with red spots, which may be accompanied by burning, swelling and pain. It is recommended that patients with itchy red spots on the knees go to the hospital, complete the relevant examinations and clarify the cause of the disease.