The Rhesus factor, also known as the Rh factor, gets its name from experiments conducted in 1937 by scientists Karl Landsteiner and Alexander S. Weiner. These revolutionary case studies involved rabbits which, when injected with the Rhesus monkey's red blood cells, produced an antigen present in the red blood cells of many humans. The Rhesus factor is an antigen, or more specifically a protein, that exists on the surface of red blood cells.

There are four general categories of blood: A, B, O, and AB. Each blood type is further labeled as positive or negative, which is a reference to the Rhesus factor of the blood. People with the Rhesus factor, that is, people with the antigen present in their blood, are Rh-positive. So a person who has a blood type of A and has the Rhesus factor is said to have A-positive, or A+, blood. More than 85% of people are Rh-positive. People without the Rhesus factor, that is, people that don't have the antigen in their blood, are Rh-negative.

A person's Rh type is generally significant only with respect to pregnancies. Specifically, a Rh-positive child born to an Rh-negative woman runs the risk of developing Rh disease. Only Rh-negative women risk having children with the Rhesus factor disease; Rh-positive women do not. So, for a Rh-negative woman to have a Rh-positive child, the father must have been Rh-positive. A Rh-positive man has a 50% chance of passing on his Rh-positive blood type to the child.

If the mother is Rh-negative and the child is Rh-positive, and the child's blood enters the woman's bloodstream during the pregnancy, labor, or delivery, the woman's immune system may 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 may naturally produce antibodies which 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, brain or heart damage, and in severe cases Rh disease can be fatal.

The Rhesus factor is less likely to affect the first-born child because the woman's system will have had less time and is therefore less likely to produce the antibodies to fight off the antigens in the child's Rh-positive blood. If the woman has become sensitized, that is, her immune system responded by producing antibodies, those antibodies will be present in her system for the rest of her life. Therefore, disparate Rhesus factor types between a woman and her child can increase the potential for Rh disease in each subsequent pregnancy.

Preventative measures to protect against the Rhesus factor disease and their devastating effects are available. Women should be tested early in their first pregnancies to determine whether they are Rh-negative and whether they are sensitized. Sensitization may not only occur through normal pregnancies, but at any time a woman and her child's blood mix, including miscarriages, ectopic pregnancies, and blood transfusions.

If a woman is Rh-negative, and has not yet been sensitized, she will likely be given an injection of a blood product known as Rh immunoglobulin (Rhlg) at about seven months into the pregnancy. This should prevent sensitization for the rest of the pregnancy. The Rhlg shot seeks to destroy any antigens present in the bloodstream before the mother is able to create antibodies. Additionally, it is generally recommended that the newborn be tested for his or her Rhesus blood type. If the child is positive, the mother is often given another Rhlg shot shortly after birth to prevent her from becoming sensitized. Rhlg injections only last for a given pregnancy. Subsequent pregnancies will likely require separate Rhlg injections.

If the woman is Rh-negative and has been sensitized, the Rhlg injection will not help. Close monitoring of the baby should be conducted to ensure that Rh disease is not developing. Blood transfusions to replace the diseased blood with healthy blood may be given during or after delivery depending on the circumstances.