Antigens are substances, typically proteins or carbohydrates, found on cell surfaces. The body’s immune system recognizes these. It identifies its own antigens as “self,” allowing them to circulate without an immune response. Conversely, the immune system targets and eliminates anything it perceives as “non-self” or foreign.
The ABO Blood Group System
The primary method for categorizing human blood involves the ABO system, relying on the presence or absence of specific carbohydrate antigens on red blood cells. These are the A and B antigens, which determine an individual’s blood type.
For instance, Type A blood has the A antigen on red blood cells and anti-B antibodies in plasma, which react against B antigens. Similarly, Type B blood has the B antigen on red blood cells and anti-A antibodies in plasma.
Type AB blood has both A and B antigens, with neither anti-A nor anti-B antibodies in plasma. Conversely, Type O blood has neither A nor B antigens on red blood cells, but contains both anti-A and anti-B antibodies.
The Rh Factor
Beyond the ABO system, the Rh factor is another significant antigen system, named after the Rhesus monkeys where it was first identified. The D antigen is the most important, determining if a person is Rh-positive or Rh-negative.
An individual is Rh-positive if the D antigen is present on their red blood cells; they are Rh-negative if it is absent. This Rh status combines with the ABO type for a complete blood classification, such as A-positive (A+) or B-negative (B-).
Blood Transfusions and Compatibility
Understanding these antigens is important for safe blood transfusions. When incompatible blood is received, the immune system recognizes donor red blood cell antigens as foreign. Recipient antibodies then bind to these foreign antigens.
This binding triggers agglutination, where red blood cells clump. This clumping can block blood vessels, damage organs, and lead to a life-threatening reaction. Careful cross-matching prevents such adverse events.
O-negative blood is the “universal donor” because it lacks A, B, and Rh (D) antigens, provoking no immune response in recipients of any ABO or Rh type. AB-positive individuals are “universal recipients” because their red blood cells possess A, B, and D antigens, and their plasma lacks anti-A, anti-B, and anti-Rh antibodies, allowing them to receive blood from any ABO or Rh type.
Antigens and Pregnancy
Antigens also play a significant role in pregnancy, particularly concerning Rh incompatibility between mother and fetus. If an Rh-negative mother carries an Rh-positive fetus, her immune system can be exposed to fetal red blood cells, often during childbirth or complications like miscarriage. This initial exposure can cause the mother to produce anti-Rh antibodies.
The first Rh-positive pregnancy poses little risk because antibody production occurs after birth. However, in subsequent Rh-positive pregnancies, if the mother produced anti-Rh antibodies, these can cross the placenta. The antibodies then attack fetal red blood cells, leading to Hemolytic Disease of the Fetus and Newborn (HDFN), causing severe anemia or jaundice in the baby.
This risk can be mitigated with preventative treatment. Rh-negative mothers receive RhoGAM injections, containing anti-Rh antibodies, around 28 weeks of pregnancy and within 72 hours after birth. This prevents the mother from forming anti-Rh antibodies if exposed to fetal Rh-positive blood.