Rh Sensitization and Pregnancy

Your Blood Type

Red blood cells (RBCs) are especially important to the body because they carry oxygen from the lungs to the rest of the body, and remove carbon dioxide waste. On the surface of each RBC is a set of proteins. Which proteins you have is determined genetically.

When doctors talk about these proteins, they refer to them as your "blood type." Everyone has a blood type. One familiar set of proteins used in blood typing refers to the A, B, O system. Your blood type may be A, B, AB, or O.

A second familiar blood typing system describes the presence or absence of Rh protein. (It's called "Rh" because it was first noticed in Rhesus monkeys.) If you have the Rh protein, you are Rh-positive. If you don't have it, you are Rh-negative. Everyone is either Rh-positive or Rh-negative.

Your Immune System

Your immune system functions to protect your body from infection. In order to do that, it must be able to tell when something doesn't belong in your body, then get rid of it. The immune system uses a complex system to identify what is "self" (or what's supposed to be there) and what is "nonself" (or what is foreign and probably dangerous). Anything that the immune system identifies as foreign is called an antigen. To protect the body, the immune system marks all antigens to be destroyed.

When your immune system detects an antigen, it produces an antibody specific to that antigen. Antibodies circulate in your blood and destroy antigens. After the initial exposure, they can quickly recognize their specific antigen and react to have it destroyed if it reappears. Antibodies speed up the immune system's response to infection. Vaccines work on this principle that the immune system produces antibodies in response to a small exposure to an antigen.

Your Immune System and Your Blood Type

In the blood, your immune system uses the proteins on the surface of the RBCs to identify the cells that belong in your body. That's why your blood type is important during transfusions, because the immune system will destroy any blood that you receive that it doesn't recognize. An incompatible blood transfusion can result in severe illness and even death. If you receive a transfusion of compatible blood, your immune system can't tell the difference between the blood you received and the blood your body produced.

Rh-Sensitization

Exposure to an incompatible blood type causes the body to produce antibodies. When an Rh-negative person is exposed to Rh-positive blood, his or her body produces antibodies to the Rh factor in the blood. This reaction is called "Rh sensitization." It is important for Rh-negative women because it affects pregnancy.

Hemolytic Disease of the Newborn

During pregnancy, the mother's antibodies cross the placental barrier and enter the blood of the fetus. If the mother is Rh-negative and has been Rh-sensitized through exposure to Rh-positive blood, she has antibodies to the Rh factor. If her baby is Rh-positive, the mother's antibodies attack the fetus' RBCs, and the baby is born with a disease called "hemolytic disease of the newborn" (or HDN), or erythroblastosis fatalis.

The symptoms of HDN result from a low RBC count. With fewer RBCs, the infant's blood has a lowered ability to carry oxygen to the body. The newborn's body tries to compensate for its anemic condition and tries to produce a large number of RBCs in a short amount of time, creating other problems in the body, especially the liver. The liver becomes overworked in trying to keep up with the production of new RBCs and the cleaning away of the dead blood cells. It becomes enlarged and unable to keep up. The dead blood cells form bilirubin and result in jaundice. Fluid also begins to accumulate in the stomach, lungs, and around the heart, giving the baby a swollen look. HDN may be fatal in the most severe cases. All of these symptoms are a direct result of the mother's antibodies attacking the infant's red blood cells.

Prevention

Preventing Rh sensitization, and thus preventing HDN, is a part of routine prenatal care. Sensitization can be prevented by giving the Rh-negative woman an injection of Rh Immune Globulin (RhIg), such as Rhophylac, at the twenty-eighth week of pregnancy and another 72-hours after delivery. An RhIg preparation is also given after any possible exposure to Rh-positive blood.

Exposure to Rh-positive blood can occur not only through incompatible transfusions, but also through carrying an Rh-positive baby. Blood may mix in the course of pregnancy during delivery, miscarriage, abortion, Ectopic (or tubal) pregnancy, amniocentesis, abdominal trauma, external manipulation of the uterus such as trying to turn a breech baby, or even across the placenta. In any pregnancy in which an Rh-negative woman carries an Rh-positive baby, there is a risk of Rh exposure and sensitization. If the father is known to be Rh-negative, there is no danger of HDN or Rh sensitization because the fetus will also be Rh-negative.

There is little risk of a woman becoming Rh-sensitized and then affecting her fetus during her first pregnancy. RhIg injections are given after each delivery to protect future pregnancies. The RhIg works to clear away any antigenic cells that may have entered the mother's body, preventing antibody-formation.

Standard prenatal care involves the testing of a woman's blood for Rh antibodies. If she is found to be Rh-sensitized already, no RhIg will be administered because it will not have any effect. Upon delivery, the baby will be watched closely for symptoms of HDN and will be treated accordingly.