ABO blood group system

Lore & Background

His third group, C, lacked both antigens, but the presence of both anti-A and anti-B in what is now group O was not fully established until later work by von Dungern and Hirszfeld. The following year, his students Adriano Sturli and Alfred von Decastello discovered the fourth type, later named AB.

Reader's Guide

The ABO blood group system is foundational to transfusion medicine. A mismatch can cause a potentially fatal adverse reaction, though such mismatches are rare in modern practice. Early classification confusion arose from competing numerical systems by Jan Janský and William L. The ABO gene on chromosome 9 encodes a glycosyltransferase that modifies red cell antigens. The rare Bombay phenotype (hh) produces antibodies against A, B, and O groups, limiting transfusions to other hh individuals. Subgroups exist, notably A1 and A2, which are not always interchangeable. ABO blood types are also present in other primates.

Did You Know?

• The ABO system is the most important of 43 blood type systems recognized by the ISBT as of the last widely known update in 2023.
• The cis-AB phenotype has a single enzyme that creates both A and B antigens.

From Homogeneous Assumption to Component Precision

For much of medical history, physicians operated under the mistaken belief that blood was a single, undifferentiated fluid. This assumption carried a deadly cost: patients routinely perished after receiving incompatible blood, simply because no one understood that the liquid contained distinct functional elements. The breakthrough came when science revealed the separate roles of each component. Red blood cells carry hemoglobin and deliver oxygen to every tissue in the body. Plasma, the pale yellow liquid fraction, serves as a chemical buffer and transports essential proteins. Platelets form the clotting machinery that stops hemorrhage. White blood cells, part of the immune defense, are almost never transfused because granulocyte therapy has only narrow applications. Modern practice therefore isolates and administers only the specific fraction a patient needs—red cells, plasma, platelets, or clotting factors—rather than pouring in an entire unit of whole blood. The one notable exception is the trauma bay, where whole blood has re-entered the clinical toolkit for rapid resuscitation.

The Clinical Thresholds of Red Cell Transfusion

The decision to transfuse red blood cells has shifted dramatically over time. Evidence accumulated showing that this liberal approach actually harmed patients, so the accepted trigger was lowered to a hemoglobin range of 70 to 80 g/L, a threshold now associated with better overall outcomes. Patients showing poor oxygen saturation may require additional units. The caution against giving large volumes stems from data demonstrating that excessive transfusion worsens outcomes. When anemia is driven by iron deficiency and the cardiovascular system is stable, oral or intravenous iron supplementation is the preferred route on both efficacy and safety grounds. Fresh frozen plasma addresses clotting-factor deficiencies, while platelet products are reserved for patients with dangerously low platelet counts.

The Global Architecture of Blood Donation

Blood for transfusion comes from two sources: the recipient's own previously collected blood (autologous) or another person's donation (allogeneic). The latter is far more common in everyday practice. In high-income settings, donors typically give whole blood intravenously, mixed with an anticoagulant, and the donation is anonymous to the eventual recipient. Yet every unit remains individually traceable through the entire pipeline—collection, testing, component separation, storage, and final administration—so that any suspected disease transmission or adverse reaction can be investigated. The picture changes sharply in developing regions, where replacement donors and paid donors still play a major role, driven by cultural beliefs and the persistent risk of infection transmission. Research into what drives people to sit in a donation chair points to prosocial motives—altruism, selflessness, charity—while the principal deterrents are fear, institutional distrust, and, in some historical contexts, experiences of racial discrimination.

Processing, Storage, and the Safety Net

Once collected, blood is not simply stored as a single bag. Centrifugation separates it into red cells, plasma, and platelets, each with its own storage rules: red cells at 1–6 °C, platelets at 20–24 °C under continuous gentle agitation, and plasma frozen at −18 °C or below to preserve its clotting factors. Plasma can be further refined into albumin, clotting-factor concentrates, cryoprecipitate, fibrinogen concentrate, and immunoglobulins. A more complex procedure called apheresis allows a donor to give a single component directly. The World Health Organization mandates that every unit be screened for transfusion-transmissible infections—HIV, hepatitis B, hepatitis C, syphilis, and, where relevant, Chagas disease and malaria. Ten countries still lack the capacity to test for all of these, often because testing kits are simply unavailable. ABO and Rh typing is compulsory before any transfusion. In some nations, platelet products receive additional bacterial screening because their room-temperature storage makes them more vulnerable to contamination.