Blood is categorized based on the presence or absence of specific proteins, known as antigens, located on the surface of red blood cells. While the ABO system is the most known, the Rhesus (Rh) blood group system is the second most important, involving over 50 different antigens. The term “Rh factor” refers specifically to the D antigen, which is the most significant and highly capable of triggering an immune response. The presence or absence of this single D antigen determines whether a person’s blood type is considered positive or negative. Understanding this factor is particularly relevant in medicine, especially concerning pregnancy management.
Defining Rh Positive and Rh Negative
The Rh factor is determined by the presence or absence of the D antigen on the surface of red blood cells. An individual is classified as Rh positive if the D antigen is present, accounting for approximately 85% of the population. Conversely, an individual is Rh negative if this specific surface protein is absent. This distinction is significant because Rh-negative individuals do not naturally carry antibodies against the D antigen.
If an Rh-negative person is exposed to Rh-positive blood, their immune system recognizes the D antigen as a foreign invader. This exposure initiates an immune response, leading to the creation of anti-D antibodies. The Rh system generally only becomes a concern when an Rh-negative person receives Rh-positive blood or during pregnancy.
How Maternal Sensitization Occurs
Rh incompatibility occurs when an Rh-negative mother carries a fetus that inherited the Rh-positive status from the father. Although the mother’s and baby’s circulatory systems remain separate during pregnancy, a small amount of fetal red blood cells can enter the maternal bloodstream, often during the birthing process.
Sensitization occurs when the mother’s immune system encounters these foreign Rh-positive fetal cells. The mother’s body activates B-lymphocytes, which begin producing anti-D antibodies. The first Rh-positive pregnancy is usually unaffected because this initial immune response is slow, generating antibodies late, typically after delivery.
Sensitization can also be triggered by events such as miscarriage, abortion, ectopic pregnancy, abdominal trauma, or invasive procedures like amniocentesis. Once sensitized, the mother’s immune system retains memory cells, ready to mount a rapid and powerful antibody response in future pregnancies. The antibodies produced are Immunoglobulin G (IgG), which are small enough to cross the placenta into the fetal circulation.
Effects on Fetal Health
If a sensitized Rh-negative mother carries a subsequent Rh-positive fetus, the circulating maternal IgG antibodies pose a direct threat. These antibodies cross the placental barrier and attach to the fetus’s Rh-positive red blood cells. The immune system then marks these coated fetal cells for destruction, a process called hemolysis.
The resulting condition is known as Hemolytic Disease of the Fetus and Newborn (HDFN), which ranges in severity. The rapid destruction of red blood cells causes fetal anemia, as the baby’s body cannot compensate quickly enough. Severe anemia can lead to heart failure and hydrops fetalis, characterized by widespread fluid retention and organ failure, which can be fatal.
After birth, the breakdown of red blood cells produces high levels of bilirubin, resulting in severe jaundice. If bilirubin levels become excessively high, the substance can cross the blood-brain barrier and cause permanent brain damage, known as kernicterus. Before preventative measures were introduced, HDFN was a major cause of infant illness and death.
Medical Prevention and Management
The medical intervention that has dramatically reduced the incidence of HDFN is the administration of Rh immune globulin (RhIg), often known as RhoGAM. RhIg is a passive antibody solution derived from human blood plasma. It works by binding to and eliminating any Rh-positive fetal red blood cells that have entered the mother’s circulation before her immune system can detect them.
By clearing these foreign cells, RhIg prevents the mother’s immune system from initiating its own long-lasting antibody production. A standard preventative dose is administered to all Rh-negative pregnant individuals around the 28th week of gestation. A second dose is given shortly after delivery if the newborn is confirmed to be Rh positive.
RhIg is also necessary following any potential sensitizing event, such as a miscarriage, amniocentesis, or abdominal trauma. For pregnancies already complicated by maternal sensitization, management involves careful monitoring of the fetus for signs of anemia. In severe cases, the fetus may require life-saving intrauterine transfusions to replace the destroyed red blood cells.