The Rhesus (Rh) factor is a protein found on the surface of red blood cells. Its presence or absence classifies an individual’s blood as either Rh-positive or Rh-negative. This classification is a significant component of a person’s complete blood type, alongside the ABO system. Understanding the Rh factor is important as it influences various biological processes, particularly in medical contexts.
The Rh Factor Explained
The Rh factor is an antigen, a substance that can trigger an immune response. If your red blood cells carry this protein, you are Rh-positive, which is the more common status. Conversely, if your red blood cells lack this protein, you are Rh-negative. This distinction carries important implications for blood compatibility.
The discovery of the Rh factor dates back to the early 20th century, involving research with rhesus monkeys. Scientists Karl Landsteiner and Alexander Wiener identified an antigen in rhesus monkey red blood cells that was also present in many human red blood cells. This shared characteristic led to the naming of the Rh factor. Understanding the Rh factor became foundational for safe blood transfusions, preventing adverse immune reactions.
Genetic Inheritance of the Rh Factor
The Rh factor is inherited through specific genes from parents. The RHD gene, located on chromosome 1, primarily determines if someone is Rh-positive or Rh-negative. A functional RHD gene results in the production of the RhD protein, making an individual Rh-positive.
The RHD gene follows a dominant inheritance pattern. If a person inherits at least one functional copy of the RHD gene from either parent, they will be Rh-positive. Conversely, an individual is Rh-negative only if they inherit two non-functional copies of the RHD gene, one from each parent. This recessive pattern explains why Rh-negative status is less common globally. Another gene, RHCE, also contributes to the Rh blood group system by coding for other related Rh proteins, but RHD is the primary determinant for Rh-positive or Rh-negative classification.
Rh Factor in Pregnancy: Potential Complications
The Rh factor is particularly significant during pregnancy due to potential incompatibility between the mother and fetus. Rh incompatibility arises when an Rh-negative mother carries an Rh-positive baby. During pregnancy or childbirth, some of the baby’s Rh-positive red blood cells can cross into the Rh-negative mother’s bloodstream.
When exposed to the baby’s Rh-positive red blood cells, the mother’s immune system recognizes them as foreign. In response, her body produces Rh antibodies, designed to target and destroy the foreign Rh-positive cells. This initial exposure usually does not harm the first Rh-positive baby because antibodies develop slowly. However, these antibodies remain in the mother’s system.
In subsequent pregnancies with an Rh-positive baby, pre-existing Rh antibodies in the mother’s bloodstream can swiftly cross the placenta. These antibodies then attack the baby’s Rh-positive red blood cells, leading to their destruction. This condition is known as hemolytic disease of the newborn (HDN) or erythroblastosis fetalis. HDN can cause health issues in the baby, ranging from mild anemia and jaundice to severe complications like hydrops fetalis (severe swelling, heart failure) and even stillbirth.
Preventing and Treating Rh Incompatibility
Modern medicine offers effective strategies to prevent and manage Rh incompatibility during pregnancy. The primary preventive measure for Rh-negative mothers is Rh immune globulin, commonly known as RhoGAM. This injection contains pre-formed Rh antibodies that work by “blinding” the mother’s immune system. When the mother receives RhoGAM, these injected antibodies destroy any fetal Rh-positive red blood cells that may have entered her bloodstream before her own immune system can recognize and produce longer-lasting antibodies.
RhoGAM is typically administered to Rh-negative mothers at specific times during pregnancy, usually around 28 weeks of gestation, and again within 72 hours after delivery if the baby is Rh-positive. It is also given after any event that could lead to mixing of maternal and fetal blood, such as miscarriage, abortion, ectopic pregnancy, amniocentesis, or abdominal trauma. For newborns already affected by HDN, treatment options depend on the severity of the condition. Mild cases may only require phototherapy, which uses special lights to break down bilirubin, a byproduct of red blood cell destruction that causes jaundice. More severe cases might necessitate blood transfusions, including intrauterine transfusions before birth or exchange transfusions after birth, to replace the baby’s damaged red blood cells with healthy ones.