More than a century ago, Dr. Karl Landsteiner’s groundbreaking discovery revolutionized medicine. He revealed that not all red blood cells are created equal. Mixing different blood types often resulted in clumping and cell destruction, a crucial finding that paved the way for understanding blood groups. This work led to the identification of the ABO blood group system, earning Landsteiner the 1930 Nobel Prize in Physiology or Medicine and transforming blood transfusions from a risky procedure into a life-saving practice.
While many are familiar with the classic A, B, AB, and O blood types, this is merely the tip of the iceberg. The reality is far more intricate. The number of distinct blood types, depending on the level of detail considered, easily reaches into the hundreds, and this number continues to grow as research progresses. So, how can there be so many variations?
The key lies in the proteins and sugars, known as antigens, residing on the surface of red blood cells. These antigens act as identification markers, creating a unique combination for each individual. This unique combination defines their blood type. Landsteiner’s ABO system classifies blood based on the presence or absence of A and B antigens. Inheritance plays a critical role; each parent contributes one copy of the gene determining ABO type. The O gene is recessive, meaning two copies are needed for an O blood type. A and B genes are dominant, overriding the O gene. Thus, inheriting one A gene results in type A blood, one B gene in type B, and both A and B genes in type AB blood.
Dr. Emily Coberly, divisional chief medical officer at the Red Cross, explains the significance of ABO compatibility in transfusions: “The ABO blood group system is the most important to consider during a blood transfusion. We all make antibodies against the ABO antigens not present on our own red blood cells.” This means type A blood contains antibodies that attack B antigens, highlighting the critical need for compatible blood types during transfusions.
Beyond ABO, another crucial antigen is the Rh factor. Individuals possessing the Rh factor have Rh-positive blood; those without are Rh-negative. While the genetics of the Rh factor are more complex than the ABO system, being Rh-positive is dominant and more prevalent. The combination of ABO and Rh factor creates eight major blood groups. While this usually suffices for safe transfusions, additional complexities exist.
The true complexity arises when considering the hundreds of other antigens present on red blood cells. As of 2024, the International Society of Blood Transfusion recognizes 47 blood group systems, each potentially encompassing numerous blood types. Examples of rarer blood types include the McLeod phenotype (lacking the Kx protein), the Kidd-null phenotype (lacking Kidd proteins), and the Bombay phenotype (lacking the H antigen). These variations are often genetically determined, sometimes associated with specific health conditions and ethnic backgrounds.
The implications of this complexity are profound, especially for individuals with diseases like sickle cell disease (SCD). Patients with SCD frequently require transfusions, sometimes leading to the development of antibodies against numerous donor blood antigens, even with ABO and Rh matching. This necessitates meticulously matched blood to prevent life-threatening complications.
The sheer diversity of blood types underscores the critical need for diverse blood banks. Dr. Coberly emphasizes: “Maintaining a diverse blood supply from donors of all backgrounds ensures blood availability for all patients, including those requiring closely matched blood.” Research into universal donor blood, either through growing red blood cells or antigen removal, offers hope for future solutions to these complexities. The ongoing quest to understand the intricacies of human blood types continues to reveal new insights, driving innovation in transfusion medicine and our understanding of human genetics.
