It is often during their first pregnancy that women first hear of irregular agglutinins. The search for irregular agglutinins or RAI is indeed part of pregnancy monitoring. The objective: to prevent fetal-maternal immunization, a complication which can occur in the event of rh incompatibility.

Agglutinins are antibodies, that is to say molecules produced by the immune system to destroy agents foreign to the body and therefore considered a threat. Irregular agglutinins are more specifically antibodies directed against molecules located on the surface of red blood cells: antigens. The risk is that these irregular agglutinins turn against the red blood cells of the individual (in the event of a blood transfusion, for example) or of the fetus during pregnancy, when the future mother is rh negative and the baby rh positive.

In pregnant women, we look for irregular agglutinins in order to detect a possible rh incompatibility between the mother-to-be and her baby. This check for irregular agglutinins, or RAI, is one of the mandatory examinations prescribed during the first prenatal consultation. It is carried out by a simple blood test. The result is expressed in positive or negative. In the event of a positive result, an identification and titration of the antibodies will be carried out in order to precisely determine the type of antibody and the molecules targeted.

As a reminder, the blood group consists of a letter – A, B, AB or O – and Rhesus, + or -. This rhesus refers to a biological particularity: in the case of rhesus positive (Rhesus-D positive or RH (1) in scientific language), the red blood cells carry on the surface a molecule called the D antigen or rhesus molecule. This is the case for 85% of the French population. In the case of rhesus negative (Rhesus-D negative or RH (-1)), the red blood cells do not have this D antigen. This is the case for the remaining 15%.

If the mother-to-be does not have a complete blood group card, during the first prenatal consultation, a double blood group determination (ABO and complete Rhesus and Kell phenotypes) is mandatory. This is important in order to detect situations at risk of rhesus incompatibility.

We speak of rh incompatibility not when the two parents do not have the same blood type, but only in this case: the mother is rh negative and the father rh positive. The baby can then inherit the positive rh from his father.

However, if blood from the rh positive fetus passes into the maternal circulation (rh negative), fetal-maternal immunization (or anti-Rhesus-D or “RhD” alloimmunization) takes place. The mother’s immune system turns against that of the fetus whose D antigens it does not recognize. The pregnant woman will then produce antibodies against these D antigens within 72 hours: these are irregular agglutins. By attacking the D antigen, they simultaneously destroy the red blood cells of the fetus, whose function is to transport oxygen. The fetus is therefore at risk of hemolytic anemia which can have serious repercussions on its growth and even its survival. At birth, the newborn may suffer from Rhesus hemolytic disease as well as severe jaundice (jaundice).

Depending on the stage of pregnancy, the risk of fetal-maternal immunization is different:

• during the first pregnancy, the placenta normally acts as a “barrier” between the two blood circulations, so there is no contact between the blood of the pregnant woman and that of the fetus, and therefore no danger of fetal immunization. – kindergarten. Except in certain specific situations: amniocentesis, shock on the stomach leading to bleeding, miscarriage leading to bleeding, hemorrhage during pregnancy;
• During the first delivery, at the time of delivery, the placenta releases the baby’s red blood cells which pass into the maternal bloodstream. The maternal immune system will produce anti-D agglutinins which will persist in the maternal blood. This does not pose a problem for the baby since it is born;
• in a subsequent pregnancy, anti-D agglutins, still present in the mother’s blood, can in certain situations cross the placenta and attack the red blood cells in the fetus.

Preventing fetal-maternal immunization begins with the detection of rhesus incompatibility situations by establishing the maternal blood group as early as possible during pregnancy – and if possible before, during a preconception visit.

It is then based on the search for irregular agglutinins (RAI) from the first prenatal visit for all pregnant women, then regularly during a second pregnancy (and the following ones) for pregnant women with rh negative.

Prevention finally involves the injection of an anti-rhesus serum or anti-D immunoglobulins in all situations where, in a rhesus negative woman, fetal and maternal blood are likely to come into contact. This injection will quickly destroy the fetal red blood cells passed into the maternal circulation, thus countering the defense response of the maternal immune system. It is performed within 72 hours in all situations favoring the passage of fetal red blood cells into the maternal circulation:

• childbirth, whatever the mode;
• termination of pregnancy, regardless of the mode;
• any spontaneous miscarriage;
• an ectopic pregnancy;
• a version of the baby by external maneuvers (VME) in case of breech presentation;
• abdominal or pelvic trauma (regardless of the term of pregnancy);
• abdominal or pelvic surgery (regardless of the term of pregnancy);
• an egg sample;
• an amniocentesis;
• genital bleeding
• strapping.

Thanks to this prevention, cases of fetal-maternal immunization and hemolytic disease of the newborn are infrequent. In the future, prevention could be even more targeted thanks to a recent technique allowing a simple maternal blood test to determine the baby’s rh, from 11 WA.