A nodule is a limited, substantial damage to the dermis or subcutaneous tissue, which may appear as a hard projection with many different shapes, such as round, oval, or blunt. There are many causes of nodules, often due to infections, but also due to vascular lesions, skin tissue hyperplasia, etc. In addition, some autoimmune diseases can also cause the occurrence of nodules: 1, infectious diseases: bacteria, viruses, parasites and other pathogens infect the body, may induce the production of nodules. For example, nodular eczema, syphilis, leprosy, sporotrichosis, cysticercosis, cutaneous pig cysticercosis, etc.; 2, vascular lesions: common, such as nodular vasculitis, the onset of the skin may also appear in the calf or foot nodules; 3, tissue proliferation lesions: such as cutaneous lymphocytoma, can also induce the appearance of nodules. If an immune reaction occurs locally under the influence of stimulating factors, the skin surface may show single or multiple scattered nodules; 4, immune diseases: common rheumatoid arthritis, rheumatoid arthritis, patients often appear nodules in the affected joints, generally no pain when pressed, and a hard texture. It can also be accompanied by joint stiffness, swelling and other uncomfortable manifestations; 5. Metabolic diseases: common in gout, gout nodules can also be called gout stones, which is one of the typical symptoms of gout patients, mainly manifested as nodule-like elevations of different sizes, which can have white material outflow after breaking. In addition, patients with xanthomatosis can also trigger the appearance of nodules on the affected skin.