“Humans have more than four blood types.”

Many people commonly believe that there are only eight human blood types: A+, A-, B+, B-, AB+, AB-, O+, and O-. This widespread understanding stems from the critical importance of the ABO and Rh blood group systems in determining compatibility for blood transfusions. Since these eight types cover the vast majority of transfusion needs and risks, they are the ones most frequently discussed and tested for in medical settings, leading to the misconception that they are the only types that exist.

However, the reality of human blood diversity is far more complex. Beyond the familiar A, B, and Rh antigens, the International Society of Blood Transfusion (ISBT) officially recognizes a total of 48 distinct human blood group systems. These systems involve over 300 different antigens that can be present on the surface of red blood cells. While these additional antigens are often less immunogenic than A, B, or Rh, their presence or absence creates a multitude of rare blood types beyond the commonly known eight.

The belief that only a handful of blood types exist persists largely due to the practical focus of medicine on the most clinically significant systems. For routine transfusions and organ transplants, matching for ABO and Rh compatibility is paramount to prevent severe, life-threatening reactions. While other blood group systems are crucial for understanding genetic inheritance, forensic science, and for patients with very specific or rare transfusion needs, they do not enter (Review) public consciousness as readily as the fundamental eight types.