Illustration of what causes pruritus, a common symptom including an itching sensation in the skin, most commonly caused by skin lesions. The main agent responsible for itching is histamine, released by basophil granulocytes (cells that flow in the blood and contribute to the immune system). On the left, healthy skin with disease agents on the surface that penetrate the skin. On the right, mastocytes that have migrated from the blood vessel to the epidermis, causing histamine to be secreted as part of the immune reaction, which produces local inflammation with swelling, red patches and itching. Histamine heads to the blood vessel in order to rally other mastocytes to the site.