The pathogenesis of skin allergy and syphilis are completely different. Skin allergy is caused by allergens, while syphilis is caused by the syphilis spirochete and is a serious sexually transmitted disease. Syphilis can be categorized into stage I and II, and even stage III syphilis. Stage I syphilis often manifests itself as an ulcer or chancre in the genital area, which manifests itself in the form of a drop-like container, forming an ulcer that is hard, similar to cartilage-like hardness, with a cleaner and often plasmatic base; the laboratory often diagnoses syphilis by dark-field testing of the lesions of the chancre, to probe for the presence of the spirochete syphilis; syphilis, which sometimes occurs in the genital area, often has to be associated with allergic The skin disease of syphilis sometimes occurs in the genital area, and often has to be distinguished from allergic skin diseases, such as fixed drug rash; patients with fixed drug rash are often allergic to medications, and their preferred site is also in the genital mucous membranes, with purplish-red vesicles and ulcers dominating the manifestations; the disease is caused by oral medications, and each time it occurs in a fixed area, and there are gray-brown petechial changes after it subsides; 2. There is a difference between stage II syphilis and cutaneous urticaria, and the stage II syphilitic rash is often Prevalent in the trunk area, dominated by rose-red, painless and itchy maculopapular rash; allergic dermatosis urticaria is often irregular wind, accompanied by obvious itching; syphilis rash is often not itchy, allergic dermatosis and urticaria are often intensely itchy; in addition, the syphilis rash that occurs in the transverse part of the body is often dominated by copper-red pimples, which are often not itchy; allergic dermatosis, such as eczema and dermatitis, accompanied by obvious pimples, papules, etc. The most important difference between syphilitic rash and allergic skin diseases is whether the patient has itching or not.