Allergic purpura spreading from calf to arm can be treated by general measures, medication and other treatments. 1. General measures: patients need to rest in bed, avoid contact with allergens, at the same time need to keep the skin clean, avoid scratching, friction, etc., can be appropriate to elevate the purpura site. 2. Medication: antihistamines such as loratadine and montelukast sodium, or drugs to improve vascular permeability such as vitamin C and triclosan can be used as prescribed by the doctor, which can effectively relieve the rash. Glucocorticoids such as prednisone and dexamethasone can also be used for relief. Anticoagulants such as aspirin and dipyridamole are also prescribed for some patients. For those with significant pain, non-steroidal anti-inflammatory drugs such as ibuprofen and acetaminophen are prescribed. For poorly treated patients, immunosuppressants such as cyclophosphamide and azathioprine can be used as appropriate. 3. Other treatments: For patients with refractory purpura who are not well treated by conventional treatment and have recurrent episodes of the disease, they need to cooperate with the doctor to carry out plasma exchange therapy, which can reduce clinical symptoms and improve the prognosis. Patients with purpura are advised to consult a doctor in a timely manner, actively cooperate with the doctor to carry out standardized treatment, strengthen the management of daily life, and follow up on time.