- A blood group, also known as a blood type, refers to the classification of human blood based on the presence or absence of specific antigens and antibodies. These antigens are found on the surface of red blood cells (RBCs), while the corresponding antibodies are found in the plasma.
- The two most important systems for blood group classification are the ABO system and the Rh (Rhesus) system, both of which are critical in blood transfusions, organ transplants, pregnancy care, and forensic science.
- In the ABO system, individuals are grouped based on the presence of A and B antigens on their red blood cells. There are four main blood groups:
- Group A has A antigens on RBCs and anti-B antibodies in the plasma.
- Group B has B antigens and anti-A antibodies.
- Group AB has both A and B antigens and no anti-A or anti-B antibodies—making it the universal recipient for ABO blood types.
- Group O has no A or B antigens but has both anti-A and anti-B antibodies—making it the universal donor for red blood cells.
- The Rh system classifies blood based on the presence (Rh-positive) or absence (Rh-negative) of the Rh factor (D antigen) on red blood cells. For example, someone with A-type blood who also has the Rh factor is classified as A positive (A⁺), while someone without it is A negative (A⁻). The Rh factor becomes especially important in pregnancy, as an Rh-negative mother carrying an Rh-positive fetus may develop antibodies that attack the fetus’s red blood cells, leading to hemolytic disease of the newborn if not properly managed.
- Compatibility between donor and recipient blood types is crucial during blood transfusions to prevent immune reactions. If incompatible blood is transfused, the recipient’s immune system can attack the donor red cells, leading to potentially life-threatening complications. For this reason, blood typing and cross-matching are standard procedures in medical settings.
- In addition to the ABO and Rh systems, there are many other minor blood group systems—such as the Kell, Duffy, Lewis, and MNS systems—that can become relevant in certain clinical or immunological contexts, especially in patients who require frequent transfusions, like those with sickle cell anemia or thalassemia.
- Blood group inheritance is genetically determined and follows Mendelian principles. The ABO and Rh genes are passed down from parents to offspring, making blood typing useful in paternity testing and population genetics. The distribution of blood groups varies by ethnicity and geographical region. For example, type O is most common globally, while type B is more prevalent in Asia.
