Systemic allergic reactions almost always develop within 3 h of exposure to suspected triggers, and it is generally accepted that the faster the onset, the more severe the reaction. A significant delay in the onset of symptoms should be considered for other diseases (e.g., serum sickness). 1. Skin: Skin numbness and fever are often the earliest symptoms of a severe allergic reaction and progress to facial flushing, pruritic urticaria and M or angioedema. An estimated 70% of patients presenting with a systemic reaction have cutaneous symptoms, and in mild systemic reactions, the skin may be the only end organ involved. 2. Upper respiratory tract: In systemic reactions, acute symptoms such as runny nose, nasal congestion, sneezing and itchy nose and eyes may occur, especially in those with allergic rhinitis. Angioedema involving the uvula, tongue, larynx or pharynx may also occur, manifesting as sudden hoarseness, loss of voice and dyspnea, which may cause respiratory arrest in severe cases. 3.Lower respiratory tract:Bronchospasm and airway mucous membrane edema often occur in the lungs, which causes symptoms such as dyspnea, wheezing or chest tightness. In patients with bronchial asthma, the reaction in the lungs may be particularly severe. 4. Gastrointestinal tract:The abdominal symptoms are usually caused by intestinal mucosal edema and smooth muscle spasm, which may manifest as acute abdominal pain, nausea, vomiting or diarrhea. Occasionally, due to intestinal ischemia or intestinal infarction may lead to obvious symptoms of intestinal bleeding. 5. Cardiovascular system: Arrhythmias, including premature atrial or ventricular beats, and other atrial or ventricular arrhythmias, or myocardial ischemia, as well as palpitations, dizziness, and chest pain, may occur in the systemic response. Rarely, acute myocardial infarction may also occur, but is more common in patients with concomitant hypotension and underlying coronary artery disease. Although the use of epinephrine in severe allergic reactions may be a cause of angina pectoris and myocardial infarction, similar complications and cardiac arrest can occur in patients not on epinephrine. Hypotension is one of the most dangerous complications of severe allergic reactions. The degree of blood pressure reduction can vary from mild to severe, with mild reductions leading to dizziness and dizziness to severe cases leading to loss of consciousness and failure of the entire cardiovascular system. Mortality: According to statistics, there are approximately 1,500 deaths from severe allergic reactions in the United States each year, the most common causes being penicillin, hymenopterous insect bites and food allergies. Deaths from severe allergic reactions can occur at any age, but most of them are over the age of 10. Asthma is a clear risk factor for fatal severe allergic reactions. A fatal severe allergic reaction usually develops within 20 min of exposure to a suspected substance. The most common first symptom is acute respiratory distress or circulatory failure within a few minutes of exposure. Pathological findings at autopsy in patients with fatal severe allergic reactions show the presence of pulmonary abnormalities (pulmonary congestion in 90%; pulmonary edema and M or hemorrhage in 50%) and laryngeal edema in about one-third of patients.