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What is the Rhesus Factor?

By M. Dee Dubroff
Updated Mar 03, 2024
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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 DDD 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.

Blood Transfusions

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.

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Discussion Comments
By anon122167 — On Oct 26, 2010

what happens to the male and female baby if both father and mother are o positive or one is negative and one is positive.

By anon118281 — On Oct 13, 2010

I have a friend that isn't sure if he is the father to their one year old son. she is a- and he is o+ and the baby is a+? can anyone help me out here?

By anon103197 — On Aug 11, 2010

is it possible to have a RH negative child when both parents are positive?

By anon101484 — On Aug 03, 2010

I am not sure how this works, but I was wondering if me and my partner have anything to worry about? I am A+ Male and she is a O- female. I know if a female who is O negative has a child with a male with O positive blood then the Rh factor can be a problem but is it still a problem in my situation? If not is there other problems? I am on a slow computer so I have problems searching for things.

By juliet — On Jul 08, 2010

we had a child in 2006 and I am find it had to conceive. Last year i had a miscarriage and up until now nothing is happening. We went to the clinic and were told that I'm rh negative while my husband is positive and that it is now too late to take the injection. What do we do?

By anon83068 — On May 09, 2010

I am confused. when i was born they said i was O+, and when i was 22 i became pregnant with my first child and was told that i am actually O-. Both of my parents are O+, and I've been told that it is not possible for your rh factor to change.

Please help me to figure this out. (please know, i am an army brat, so I've been to hospitals in three diff countries. Is it possible for them to all make the same mistake? or is it really possible for my rh factor to change?)

By anon68533 — On Mar 03, 2010

to thiru: obviously she should go for RhIg injection since she is rh negative. or else if her child will be rh positive so her blood will produce antibodies against the child's antigen which will be harmful for child's health. prevention is better than cure. take care.

By anon62213 — On Jan 25, 2010

i have five children. the first and third are Rh negative, while the other three are Rh positive. My husband is Rh positive and i am Rh positive. However, i learned that i had an autotransfusion after my birth in 1952 at Manchester when i was a toddler in relation to Rhesus Factor. I don't really understand what happened.

By anon57201 — On Dec 21, 2009

Are there complications when only the woman is negative or is is vice versa? That is, what if the man is negative and the woman is positive?

By anon55041 — On Dec 04, 2009

Hi i am rh neg and had 3 miscarriages in the last two years. i am pregnant again and i read everywhere about the rhlg shot and even spoke to my doctor, but it seems that they don't want to give me the shot. i am afraid of losing this baby again.

what should i do and can my baby be born abnormal or what? Please help -- do i have to get the shot or what?

By anon53997 — On Nov 26, 2009

Can any one help in this? I have B+ve and my wife is B-ve blood group. This is her first pregnancy and she's around two months along.

Will my baby be born with problems because of this Rh factor? do i need to suggest she go for (RHiG) injection during her seventh month of pregnancy? will this rh factor affect my wife's health?

please help me in this..



By anon51253 — On Nov 04, 2009

To schu: The answer is no. A child is cannot be type o blood if either the parents are not. That's why blood types used to be a main factor in disputes of paternity when testing for DNA was not yet performed. I hoped this helped.

By MissJoy — On Oct 23, 2009

I am an A negative female and my partner is O positive. We want to have a baby, but we would also like to know everything we can about our blood types. We would also like to know if we could have more than one healthy child. Any and all answers will be much appreciated! Thank you

By chilli — On Sep 18, 2009

I am 0+ male and my fiancee is O- female. i know about the rhesus incompatibility. i would like to know if this can be completely taken care off by proper treatment.

By anon44897 — On Sep 11, 2009

the only way you could have complications is if the mother is RH - but that is why there are blood tests to determine blood type early in pregnancy so then once you reach 26-28 weeks they give you a shot to keep it from attacking the baby. it is okay to have a RH- and be pregnant.

By anon41562 — On Aug 15, 2009

what if both parents are positive(father is O and mother is B). is it possible to bear a child who is negative( B-)?

By anon41236 — On Aug 13, 2009

im rhesus d negative and my partner is rhesus o negative. would that complicate any pregnancies?

By anon41199 — On Aug 13, 2009

If o+ men marry an A- girl, is there any problem for child?

By abena — On Aug 07, 2009

Can a woman with o+ marry a man with o+ without any child bearing problems?

By anon35915 — On Jul 08, 2009

If me and my husband are A+ will there be any problems in pregnancy

By avie — On Jun 02, 2009

i'm rh negative & my husband rh positive. our first baby died; a newborn. it was rh positive. it died of a lung hemorrhage. kindly inform me if the rh factor could be a danger for the next baby. precautions recommended.

By premahuja — On Mar 26, 2009

Is it possible for a couple, both of whom are Rh+ to have Rh- child?


By satishpgoyal — On Feb 18, 2009

I have B negative bld grp and very soon going to marry. She has A positive bld grp. Plz tell me will there b any complication in our child?

By anon25317 — On Jan 27, 2009

What if the woman is rh+ and the man rh-, what is the result of their children?

By anon22904 — On Dec 12, 2008

I am O+. After I delivered my third baby, I was told that my body produced antibodies (anti-c)! Which was odd. How is that possible?

By markc — On Dec 01, 2008

Hi can anyone help? if my wife is rh o- and i am ab + is it possible to father a child with a- blood

thanks m ...

By danielfarina — On Sep 30, 2008

is it possible to change my blood type from ab+ to ab-? if so, how?

By anon16055 — On Jul 28, 2008

Can a RH-negative father and a RH-positive mother bear children without complications?

By Kris3065 — On Jul 25, 2008

I am A+, my husbands blood switches between + & -. When my son was born they had to test his blood many times and finally determined that he is a positive donor but a negative receiver. What causes this? Is it likely to cause him any problems?

By momma303 — On Apr 28, 2008

My daughter is 0- and her husband is +. She had a miscarriage about a year ago and didn't know that she should have been checked for antibodies. She is now pregnant and everything looks good, but the doctor does not want to see her until her 8 to 10 week check-up. Should she insist on being seen earlier and what are the chances that she became sensitive? Her miscarriage was @ 4 to 6 weeks.

By olivia — On Jan 30, 2008

There are numerous factors in calculating blood types and unfortunately we can't provide that service here on wiseGEEK. But, if you input "blood type calculator" into your search engine, several easy-to-use choices come up!

By malena — On Jan 27, 2008

Here are the possibilities:

If one parent is Rh positive and the other is Rh negative, the child could be either Rh positive or Rh negative. The only way to know is to test the child's blood type.

If both parents are Rh negative, the child will necessarily be Rh negative.

If both parents are Rh positive, the child could be either Rh positive or Rh negative. This is because people have two alleles going to their Rhesus factor. Rh positive people can have two Rh positive alleles, or one positive and one negative while still being classified as positive. If at least one of the positive parents has a negative allele then that negative factor can be passed on to the child. A Rh negative person, on the other hand, necessarily has two Rh negative alleles.

By justd2006 — On Jan 04, 2008

Can a mother with O- blood and a father with A positive have an O+ child? in school I was told this is not possible- is this true?

By Shayna — On Dec 17, 2007

Can two O+ parents have an O- child?

By renukrs — On Oct 14, 2007

can a woman with A1 positive marry a man with O negative? will she face any problems with during the pregnancy???

By anon1382 — On May 28, 2007

can a lady who is O negative and shorteye sighted marry a man who is o positive and also short sighted.

what is/are the possible effect on their children?

is it advicale for them to get married?

By Dayton — On May 26, 2007

Yep, that is certainly possible. If the father's blood type is AO+- (which would appear as an A+), he could give the O- to his child, who would also receive an O- from the mother. The only way to have an O- blood type is OO--.

By anon1337 — On May 25, 2007

If the mother is O- and the father is A+, is it possible to have a child with O-?

By Dayton — On May 21, 2007

Interestingly enough, that IS possible. Essentially, one parent would have to be AO+- (which would appear as A+), and the other BO+- (which would appear as B+). The child could receive an O and an - from each parent.

Genetics at work!

By schu — On May 20, 2007

If one person has A positive blood and the other has B positive blood, is it possible for their children to have O negative blood.

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