Blood type O-negative is considered the universal donor, as it can be safely transfused to people of any blood type, including A, B, AB, and O. This is because O-negative blood cells do not contain A or B antigens, which can trigger an immune response in the recipient's body. However, people with O-negative blood can only receive blood from other O-negative donors, as their immune system may react to any other blood type. Therefore, O-negative blood is in high demand for emergency transfusions and is often used when the recipient's blood type is unknown or in situations where there is not enough time to determine the recipient's blood type.