".....In the past, a person with blood type O negative blood was considered to be a universal donor. It meant his or her blood could be given to anyone, regardless of blood type, without causing a transfusion reaction. This is no longer a relevant concept because of a better understanding of the complex issues of immune reactions related to incompatible donor blood cells."
Reference: Mayo Clinic Health Oasis - Ask a Physician 08/09/2000. As quoted in: Blood types and compatibility. Bloodbook.com; 2005. Available at: http://www.bloodbook.com/compat.html. Accessed February 15, 2016.
Transfusion of blood components is generally a safe and effective way to correct hematologic deficits. However, a transfusion reaction may occur and health care providers must be aware of the risks involved with blood transfusions and evaluate the risks against the potential therapeutic benefits.
A transfusion reaction can be defined as any adverse event occurring during or after the transfusion of blood components. Adverse events can range from fever and hives to renal failure, shock, and death. Some adverse events can be prevented, but others cannot.