Implications of a "Negative" Blood Type
Blood type screening is one of the routine tests we offer to all pregnant women. Blood type is defined by the presence or absence of specific substances that are exposed on the surface of red blood cells (RBCs). At present, there are 29 different human blood group systems recognized by the International Society of Blood Transfusion. For sake of simplicity, the most significant of these are the ABO and Rh (Rhesus) blood group systems.
The ABO system defines the major blood group “antigens” (things to which the immune system can react if they are foreign to our bodies). The “O” part of this system represents, actually, the absence of either “A” or “B.” We inherit one copy of the genes for this blood group system from each of our parents and the presence of either (or both) “A” or “B” determines the individual’s blood type. Therefore, an individual can be A (= AA or AO), B (= BB or BO), AB, or O (= OO). Yes, therefore, it is possible to be an “O” blood type and have parents that are “A” or “B.” But, if both of your parents are O, and you are “A” or “B,” or if you have a parent who is AB and you are not “A,” “B,” or “AB,” then someone is not telling you something!
The Rhesus blood group system is a little more complicated and, again, for the sake of simplicity (forgive me on this one any ‘professionals’ out there), for the most part we are concerned about whether or not you have the “D” antigen of this system expressed on your RBCs. If you do have this, then you are considered to be Rh(D)-positive and if you do not, then you are Rh(D)-negative. The Rh(D) status determines whether you have a “positive” or “negative” blood type. Thus, when you combine this with the ABO typing, you are classified as one of the following: A-positive, A-negative, B-positive, B-negative, AB-positive, AB-negative, O-positive, or O-negative. In North America, about 15% of whites and about 7-8% of Blacks will be Rh-negative. But, there is dramatic worldwide and subpopulation variation on this. For example, only about 1% of Chinese and Japanese, but almost 100% of Basques, are Rh-negative.
So, why do we screen for this in pregnancy and what is our concern regarding Rh-negative women? When women are pregnant, they can be exposed to a blood type that is different than their own – the baby’s (remember, the baby is only half you, Mom); and, when our immune system is exposed to things that are foreign to our bodies (like somebody else’s blood), we can make antibodies against those things. For example, if a mother is Rh-negative and her baby is Rh-positive (thank the father of the baby), AND the mother is exposed to enough of the baby’s RBCs, she may make antibodies to the Rh(D) “antigen.” Certain antibodies (IgG) are actively transported across the placenta from the mother to the baby and provide a source of “immunity” for the baby during the first 4-6 months of life; other antibodies (IgM) are too big and cannot be transported across the placenta. Antibodies to the major blood group antigens (ABO) are usually (but not always) of the IgM class and therefore do not get to the baby or cause problems. Unfortunately, antibodies to Rh(D) are usually IgG antibodies that readily cross the placenta (with the rest of the protective IgG antibodies) and may be the source of problems, specifics of which and current management of the same, which we will discuss in subsequent posts.
When a woman develops antibodies to Rh(D), she is considered to be “sensitized” or “isoimmunized.” Rh-isoimmunization used to be a BIG problem in obstetrics. However, about 40 years ago we learned that if we gave an Rh-negative woman a small amount of the same anti-Rh(D) immunoglobulin (that we don’t want her to make on her own) during episodes of bleeding or, prophylactically, in early third trimester (around 28 weeks’) and within 48-72 hours after delivery, we could significantly reduce the risk of her becoming “sensitized” on her own, thereby, protecting the current and, especially, a future pregnancy.
So, Lynda, if you are Rh-negative and have a negative “antibody screen” (no abnormal antibodies to Rh or any other blood group system), there is nothing to worry about at this point. Your doctor will probably repeat that antibody screen around 28 weeks’ and administer “Rh-immunoglobulin” to help prevent isoimmunization during the third trimester when it is most likely to occur. (If the baby is found to be Rh-positive after delivery, you will be given the Rh-immunoglobulin again prior to discharge). Of course, if the father of the baby is also Rh-negative, then you don’t even really need that because your baby could not be Rh-positive. But, before you open that can of worms, be sure you know “who the Daddy is!”
And, in my next posts, I will continue the discussion on Rh and tell you about a patient we admitted to the hospital yesterday who had complications related to the fact that she had an Rh-negative blood type and did become sensitized during a previous pregnancy….