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RH Disease and Red Blood Cell Alloimmunization (Isoimmunization)

When your body is exposed to germs or cells that are different from your own body's normal healthy tissues, your body forms a substance called antibody (immunoglobulin) against areas on the foreign cells that are different from your own cells called antigens (antibody generator).Antibodies attach to the antigens and  destroy the invader directly , or label them for removal by your  white blood cells. The first response by your body to the unfamiliar foreign  antigen is called sensitization and your body is now prepared  to recognize and respond with greater force on the next  encounter with this same unwanted intruder. 

The surface of your red blood cells is also covered by different antigens that you have inherited from each of your parents . Red blood cells may be identified as different types and separated into groups based on the presence or absence of the antigens on the red blood cell's surface.

For example, a person who has the D antigen (also known as Rh factor) on their red blood cells is called Rh-positive People who don't have the D antigen are called Rh-negative. 

 RhD -negative  RhD positive

The instructions for making the antigens on the surface of your blood cells are stored in the form of molecules called genes. Genes are found on structures in the cells of your body called chromosomes that are inherited from each of your parents. Different versions of the same antigen may be produced by differences in the same gene.  The different versions of the antigen form a blood group. There are thirty major blood group systems currently recognized including the ABO , Rh (Rhesus) ,  Kell ,Duffy (Fya), Kidd (Jka), MNS blood group systems .

The genes that code for the different blood types (antigens) are inherited from your parents. One set of genes from each parent.  In general, a person needs only one gene for a blood antigen to produce that antigen on their red blood cells. The chances of a baby inheriting a gene for a blood antigen  depends on how many copies of the gene each parent has available to pass on to the baby.  In the example below a mother who is RhD negative has children with a father who is RhD positive . The father may have one or two copies of the RhD gene.  If the father has only one copy (heterozygous) of the D antigen gene, their children will have a 50% chance of being RhD positive.  If the father has two copies (homozygous) of the gene for the D antigen, then all of their children will be Rh-positive.


85% of the white European and north American population is RhD positive and just over half have of them have  only one copy (heterozygous)  of  the RhD gene. Approximately 9 percent of Caucasians and 2 percent of  African-Americans are Kell positive.  Nearly  all (>97%) persons who are are Kell positive have only one copy of the Kell (K1) gene [13].

If your baby inherits a blood type from the father that is different from  your own  blood type you may become sensitized to the baby's blood type and produce antibodies against the baby's red blood cells. Antibodies may be produced in five different forms. Immunoglobulin M (IgM) , and immunoglobulin G (IgG) are the forms found in the greatest amounts in your blood . IgG is the only form of antibody that can cross the placenta into the baby's blood .Women  most often become sensitized to a blood type different from their own  during a pregnancy or after a blood transfusion.  The formation of antibodies against an antigen from another human being (the same species) is called alloimmunization or isoimmunization.

More About Antibodies

There are 5 different forms (classes)  of antibodies known as  Immunoglobulin A (IgA), IgD, IgE, Immunoglobulin M (IgM) , and Immunoglobulin G (IgG) account for most of the antibodies found in your blood and are the two types of antibody of interest to us.

 Immunoglobulin M (IgM) is the first kind of antibody to be formed after you have come in contact with a germ or other  foreign  antigen for the first time.  IgM appears about 3 to 7 days after an infection . IgM may remain elevated from weeks to months after exposure to antigen. Some IgM  antibodies occur naturally and appear without previous exposure to antigen.  Cold antibodies are antibodies that attach best  to red blood cells at less than 37 degrees C (below body temperature).   Cold antibodies are mostly of the IgM type . IgM antibodies do not cross the placenta

Immunoglobulin G (IgG) usually  appears about 7 to 10 days after an infection and may remain elevated for life providing you with resistance  to infection or disease.  . Transfer of your IgG antibodies to your baby  helps to protect your baby against infections before birth and for the first months of life . IgG is the only kind of antibody that can cross the placenta into the baby's blood

Hemolytic Disease of the Fetus and Newborn (HDFN)
 If a mother has antibodies against the blood type of a baby she is carrying , the mother's antibodies may cross into the baby's blood and attach to the baby's red blood cells causing destruction of the red blood cells (hemolysis) and lead  to anemia (low red blood cells)  in the baby.

The contents of the destroyed red blood cells are released and broken down to form a yellow substance called bilirubin that causes yellowing of the baby's skin and the amniotic fluid.   This condition is called hemolytic disease of the fetus and newborn (HDFN) .

If the anemia becomes very severe, the baby will try to keep up with the blood loss by producing new blood cells in the liver. The liver may then become swollen and damaged  causing fluid to collect in the belly, chest, and under the skin of the baby . This condition is called hydrops .

HDFN is most commonly caused by a mother who does not have the D antigen on her red blood cells (Rh negative) producing antibodies against the blood of her baby that has the D antigen on its red blood cells (Rh positive)


Screening for Blood Antibodies

Testing for blood antibodies is part of usual prenatal care and is done using the  indirect Coombs test (also known as indirect antiglobulin test or IAT) .The test is commonly referred to as the "antibody screen" .  Sometimes an antibody will be found in too small of an amount to identify, and you will need to have your blood retested.. Once identified , the level of the antibody in your blood is measured and reported as a number called a titer. A low titer number means there is a small amount of the antibody in your blood. A higher titer number means you have a larger amount of the antibody in your blood.

As the amount of antibody in your blood increases the chances that your baby will have severe anemia also increases . The antibody level that severe anemia and fetal hydrops is more likely to occur is called the "critical" titer and in most centers this level is between 8 and 32.

To determine the  level of antibody in your blood a sample of your blood is diluted over and over again until the antibody is no longer  detected.  The titer is
the last dilution at which the antibody is detected. In the example below  a blood sample (tube 1) has been diluted by 1/2 to produce tube 2, tube 2 was diluted by 1/2 to produce tube 3 and so forth. A substance was added to each tube to detect antibody. The 4th tube is the last dilution at which the antibody is detected, and it is 1/8 th the strength of the original sample. The titer of the antibody is  8













Anti-D (Rho) Immunoglobulin (RhD-Ig g, Rhogam) [7]

Rh immune globulin (RhIg) anti-D (anti-Rh) is an antibody given to women who are Rh negative (do not have the D antigen on their blood cells ) to prevent sensitization of the mother to the D antigen.  Rh-immune globulin must be given before the mother begins to produce her own antibody to the Rh factor. RhiG does not prevent sensitization to any other antigen of the Rhesus blood group or the antigens from any other blood group. Rh immune globulin (RhIg) is marketed under several tradenames such as HyperRHO , MICRhoGAM,RhoGAM, Rhophylac, WinRho . Because RhIg is an anti-D antibody women who are Rh negative who have been given RhIg may have the antibody detected for up to 12 weeks after receiving the shot (and in some cases even longer). An elevated anti-D antibody  level would be expected to decrease over time if it is caused by treatment with anti-D (Rho) Immunoglobulin..

The following antibodies do not usually cause HDFN [1-3, 10)

Lewis (Anti-Lea, Anti-Leb ), anti-P1, anti-I, anti-N, anti-M , and cold antibodies (antibodies that react below body temperature , 37 degrees C)  are usually IgM antibodies that cannot cross the placenta and would not be expected to cause HDFN. some   naturally occurring since they appear without previous exposure to antigen However some of these antibodies such as anti-M may also be present in the  IgG form than can cross the placenta and potentially cause HDFN. If anti-M, IgG optimally reactive at 37 degrees C, is identified in the mother's blood, the father's blood must be checked for the presence of the M antigen.

The following antibodies usually cause no or mild anemia . Routine obstetric care is recommended by the American College of Obstetricians and Gynecologists  [ACOG]

Kell( anti-k, anti-Ko, anti-Kpa, anti-Kpb, anti-Jsa, anti-Jsb), Duffy (anti-Fyb, anti-By3), Kidd (anti-Jkb, anti-Jk3) , MNS (anti-N), Vw, Mur, Hil, Hut, Lutheran (anti-Lua, anti-Lub), Xg (anti-Xga), Public antigens (anti-Ytb, anti-Lan, anti-Ge, anti-Jra, anti-Co1-b), Private antigens ( anti-Batty, anti-Becker, anti-Berrens, anti-Evans ,anti-Gonzales, anti-Hunt, anti-Jobbins, anti-Rm, anti-Ven, anti-Wrightb)

The following antibodies may produce mild to severe HDFN. ACOG recommends fetal assessment  [1]

Rh (anti-D, anti-E, anti-c ), Kell (anti--K), Duffy (anti-Fya)  Kidd (anti--Jka), MNS (anti-M, anti-S, anti-s , anti--U, anti-Mia, anti-Mta) , Diego (anti-D1a, anti-Dib) , P (anti-P1PK also known as anti-TJa)- may cause severe HD Public antigen (anti-Yta, anti-Ena, anti-COa), Private antigens (anti-Biles, anti-Good, anti-Heibel, anti-Radin, anti-Wrighta, anti-Zd)

Rh (anti-D, anti-E, anti-c ), Kell (anti--K), Duffy (anti-Fya) antibodies are the most likely to cause HDFN requiring intrauterine transfusion.

If you have an antibody that could cause severe anemia in your baby, then the red blood cells of the father should be tested for the matching antigen to the antibody. If the father does not have the antigen on his blood cells, then the baby will not have the antigen either and repeat testing for antibodies at 28 weeks is recommended.  If the father has the antigen or the father of the baby is unavailable for testing, then the baby will need to be tested to see if it has inherited the father's antigen.

To determine the father's antigen status most laboratories will ask that you submit 5 mL of the father’s blood drawn into a Lavender-top (EDTA) tube with a request to perform Red Blood Cell Antigen Typing, and specify the bood group to be tested (Kell, Duffy, etc.)  group  on the father’s blood. If the father is negative for the antigen (and it is certain that he is the father of the fetus) further evaluation is unnecessary. If the father has the  antigen or the status of the father is unknown, then amniocentesis to determine the fetal antigen type maybe performed.


  • Except in cases of alloimmunization due to anti-Kell antibodies (and possibly anti-M IgG antibodies) , red cell antibodies in pregnancy are managed in a similar fashion.

  • Antibody titers are not used for monitoring Kell-sensitized patients because Kell antibodies levels do not predict the well being of the baby. Kell sensitized pregnancies may be followed using ultrasound as for a woman with a previously affected pregnancy.

  • Recommendations on the management of patients with anti-M IgG differ, some authorities recommend in women with a first sensitized pregnancy have levels of the antibody measured as for other blood antibodies if the first titer is greater than 4. Others suggest that serial antibody titers are not reliable, and the that pregnancies with anti-M IgG should be monitoring as Kell-sensitized pregnancies are.

Monitoring a First Sensitized Pregnancy [1,4,5,8 ]

  • If this is the first pregnancy that you have had with a positive blood antibody (and you do not have anti-Kell or anti-M antibodies) the level of the antibody in your blood will be measured  every month until about
    24 weeks  and then every 2 weeks thereafter. If the titer remains under less than or equal to 16  the critical value you may deliver at term.

    • If the titer is more than 16 an ultrasound machine will be used to see if your baby is developing anemia by  measuring the speed of the blood flowing through a blood vessel in your baby's brain called the middle cerebral artery. This measurement is called the middle cerebral artery peak systolic velocity (MCA PSV). The measurements are started at about 18 weeks and are repeated every 1 to 2 weeks. The MCA PSV is a reliable predictor of severe anemia until 35 weeks gestation.

    • If the flow of the blood becomes  faster than the expected flow for your baby's age (1.5 MoM) , severe anemia is more likely and you may be referred for blood transfusion of the baby while it is still in the womb.

    • Antenatal testing is started at 32 weeks and delivery  at 37 to 38 weeks is recommended.

Monitoring in a Woman with a Previously Affected Pregnancy [1, 3,6]

  • If you have had a previous pregnancy with stillbirth related to HDFN ,  fetal hydrops, intrauterine fetal transfusion, preterm delivery because of fetal anemia, or neonatal exchange transfusion ultrasound will be used to see if your baby is developing anemia by  measuring the speed of the blood flowing through  your baby's brain starting at about 18 weeks. The measurements are repeated every 1 to 2 weeks.

  • If the flow of the blood becomes  faster than the expected flow for your baby's age (1.5 MoM) , severe anemia is more likely and you may be referred for blood transfusion of the baby while it is still in the womb

  • Antenatal testing is started at 32 weeks and delivery  at 37 to 38 weeks is recommended.

Suggested Reading

  Reviewed by Mark Curran, M.D. FACOG


1. Management of alloimmunization during pregnancy. ACOG Practice Bulletin No. 75. American College of Obstetricians and Gynecologists. Obstet Gynecol 2006;108:457–64.PMID: 16880320
2. Thompson DJ, et al., Anti-M antibody in pregnancy Obstet Gynecol Surv. 1989 Sep;44(9):637-41. PMID: 2771305
3. Moise KJ. Hemolytic Disease of the Fetus and Newborn. In: Creasy RK, Resnik R, Iams JD, eds. Creasy and Resnik's Maternal-Fetal Medicine: Principles and Practice. 6th ed. Philadelphia, Pa.: Saunders/Elsevier; 2009:477-503.4. De
4.Wikman A, et. al. Fetal hemolytic anemia and intrauterine death caused by anti-M immunization.
Transfusion. 2007 May;47(5):911-7. PMID: 17465958
5. Yasuda H, et. al., Hemolytic Disease of the Fetus and Newborn With Late-Onset Anemia due to Anti-M: A Case Report and Review of the Japanese Literature. Transfus Med Rev. 2013 Oct 19. pii: S0887-7963(13)00065-5. doi: 10.1016/j.tmrv.2013.10.002. PMID: 24262303
6. Moise KJ Jr, Argoti PS.Management and prevention of red cell alloimmunization in pregnancy: a systematic review. Obstet Gynecol. 2012 Nov;120(5):1132-9. doi: http://10.1097/AOG.0b013e31826d7dc1.PMID: 23090532
7.  RhoGAM®  prescribing information Ortho Clinical Diagnostics Revised November 2010 
8.  Moise KJ Jr. Management of rhesus alloimmunization in pregnancy. Obstet Gynecol. 2008 Jul;112(1):164-76. PMID: 18591322
9.Gooch A, et al.,Guideline for Blood Grouping and Antibody Testing in Pregnancy. British Committee for Standards in Haematology 2008 Available at: http://www.bcshguidelines.com/documents/antibody_testing_pregnancy_bcsh_07062006.pdf
Accessed 1/12/2014
10. Guidelines for Blood Grouping and Antibody Screening in the Antenatal and Perinatal Setting. The Australian & New Zealand Society of Blood Transfusion Ltd. 2007 Standards in Haematology 2008 Available at: http://www.anzsbt.org.au/publications/documents/Antenatal_Guidelines_Mar07.pdf
Accessed 1/12/2014
  11. Pregnant Women with Red Cell Antibodies: Scottish National Clinical Guidance. Version 2, July 2013
Available at: http://www.scotblood.co.uk/media/101700/pregnant_women_with_red_cell_antibodies_scottish_national_clinical_guidance_august13.pdf Accessed: 1/12/2014
12. Zimmerman R, et. al., Longitudinal measurement of peak systolic velocity in the fetal middle cerebral artery for monitoring pregnancies complicated by red cell alloimmunisation: a prospective multicentre trial with intention-to-treat. BJOG. 2002 Jul;109(7):746-52.PMID: 12135209
13. McEwan A. Red cell alloimmunization. In Parvord S and Hunt B eds The Obstetric Hematology Manual. 1st ed ,New York Cambridge University Press; 2010 p 771
Sikkel E, et al., On the origin of amniotic fluid bilirubin.
Placenta. 2004 May;25(5):463-8.PMID: 15081641

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