Type O blood is known as the universal blood type and is usually allowed to go for blood donation. However, if type O blood is rejected, it may be due to the fact that it is a contraindicated group for blood donation, or it is at a special time when it is temporarily unsuitable for blood donation. It is recommended to ask the medical staff of the blood donation institution or to seek medical examination to rule out the existence of relevant diseases or other factors affecting blood donation, and if diseases exist they should be treated promptly. Donors are also not allowed to donate blood if they have the following conditions: 1) those who are younger than 18 years old and over 55 years old; 2) those who do not have diseases but are thin and do not reach the weight limit, such as men who weigh less than 50 kg and women who weigh less than 45 kg; 3) patients suffering from leprosy, AIDS, viral hepatitis, tuberculosis and other infectious diseases, such as hepatitis B and C; 4) severe allergic Patients with serious allergic diseases, cardiovascular diseases, urinary system, digestive system, respiratory system diseases, such as coronary heart disease, acute gastroenteritis, cold healed less than 1 week, urinary tract infection healed less than 1 month, etc.; 5. These patients are not suitable for blood donation due to the risk of infection or their poor physical condition. In addition, some specific periods, such as tooth extraction within a short period of time, during miscarriage, pregnancy, childbirth, breastfeeding and before and after menstruation, as well as major surgery for less than six months, are not recommended for blood donation and can wait until the special period has passed before donating blood.