text: The Rhesus factor, also known as the Rh factor, is an antigen that exists on the surface of red blood cells in most people. People who have the Rhesus factor are considered to have a "positive" (+) blood type, such as A+ or B+. Those who don't are considered to have a "negative" (-) blood type, such as "O-" or "AB-." The Rhesus factor gets its name from experiments conducted in 1937 by scientists Karl Landsteiner and Alexander S. Weiner. Their experiments involved rabbits which, when injected with the Rhesus monkey's red blood cells, produced an antigen that is present in the red blood cells of many humans.
  The ABO Blood Grouping System
  Although there are at least 30 different systems for grouping blood types, most people are familiar with the ABO system, which groups blood into four general types: A, B, O and AB. Each blood type is usually further labeled as positive or negative, which is a reference to the Rhesus factor of the blood. More than 85% of people are Rh+.
  The Rh Factor and Antigens
  The Rh blood grouping system actually involves more than 50 antigens that are found on the surface of red blood cells. These antigens are proteins that, when introduced into a body that does not have the same type, can cause the person's immune system to respond by producing antibodies that attack the proteins. The Rh factor, Rh+ and Rh-, usually refers specifically to the presence or absence of one of these proteins — the D antigen. The D antigen tends to cause an especially strong immune response in people who do not have it.
  There are two alleles, or genetic variants, of this antigen: D and d. A person who is Rh- has two recessive variants, dd. Anyone who has at least one D — DD or Dd — is Rh+. As with most genetic traits, one allele is inherited from each parent.
  Rh Type and Pregnancy
  A person's Rh type is generally most relevant with respect to pregnancies. During pregnancy, an Rh+ fetus developing in the womb of an Rh- woman runs the risk of developing Rhesus disease, also called Rh disease or hemolytic disease of the newborn. Only Rh- women risk having children with this disease; an Rh+ woman can carry an Rh- child without developing this condition.
  For an Rh- woman to have an Rh+ child, the father must have been Rh+. An Rh+ man has at least a 50% chance of passing on the Rhesus factor to the child; a Dd father could pass either the D or d to his child. If the father is DD, there is a 100% chance that the child will be Rh+.
  If the mother is Rh- and the child is Rh+, and if the child's blood enters the woman's bloodstream during pregnancy, labor, or delivery, the woman's immune system might respond by producing antibodies to fight off the child's antigens, which are foreign to the woman's system. That is, the woman's body might naturally produce antibodies that attack the baby's blood, causing the baby's red blood cells to break down. The result of this incompatibility will not affect the health of the mother, but it can affect the child's health. Potential health problems include jaundice, anemia, and brain or heart damage. In severe cases, Rh disease can be fatal to the infant.
  To protect itself from the rhesus factor, a Rh- woman's body usually first becomes sensitized to the D antigen. This means that her immune system has been exposed to the protein, and has started to produce antibodies to fight it. Rhesus disease is less likely to affect an Rh- woman's first-born Rh+ child, because the mother and child's blood usually does not mix until labor and delivery. At that time, the mother's body may not have had the time to make enough antibodies to cause serious problems.
  Once the woman's immune system has responded to a child's antigens by producing antibodies however, those antibodies will be present in the mother's system for the rest of her life. The potential for Rh disease increases with each subsequent pregnancy, because the antibodies will be present throughout the duration of each pregnancy after they are first produced.
  Protecting Against Rh Disease
  There are preventative measures to protect against Rhesus disease and its effects. Women should be tested early in their first pregnancies to determine if they are Rh- and if they are sensitized. Sensitization might occur not only through normal pregnancies, but any time a woman and her child's or fetus' blood mix, including miscarriages, ectopic pregnancies and blood transfusions.
  If a pregnant woman is Rh- and has not yet been sensitized, she usually will be given an injection of a blood product known as Rh immunoglobulin about seven months into the pregnancy. This should prevent sensitization for the rest of the pregnancy. The Rh immunoglobulin shot seeks to destroy any Rh+ antigens produced by the baby and present in the mother's bloodstream before the mother is able to create antibodies. Additionally, it generally is recommended that the newborn be tested for his or her Rhesus blood type.
  When the child is Rh+, the mother is often given another Rh immunoglobulin shot shortly after birth to prevent her from becoming sensitized. Rh immunoglobulin injections last only for a given pregnancy. Subsequent pregnancies will likely require separate Rh immunoglobulin injections. This treatment works to prevent Rh disease in 99% of cases.
  If the woman is Rh- and has been sensitized, the injection will not help. Close monitoring of the baby typically is conducted to ensure that Rh disease is not developing. Blood transfusions to replace the damaged blood with healthy blood might be given during or after delivery, depending on the circumstances.
  Although the Rh factor is most often discussed in reference to pregnancy, it does play a role in other health matters. Just as a woman's body can develop antibodies that attack her baby's blood, a patient who is Rh- can have a transfusion reaction — an allergic reaction to the blood — if he or she is given blood from an Rh+ positive donor. Such reactions are relatively uncommon because blood is screened for the Rh factor, and Rh- patients receive Rh- blood during a transfusion whenever possible.