Blood groups 


Blood group O is the most common blood group in the UK 

Giving blood

Most people are able to give blood but only 4% actually do. You can donate blood if you:

  • have a good overall level of health
  • are 17-65 years of age (if it's your first time)
  • weigh at least 50kg (7st 12lb) 

Take the donor health check to find out if you are eligible to give blood.

Find your nearest Blood Donor Centre in England and North Wales and book an appointment online. You can also call 0300 123 23 23 to book an appointment.

Read more about giving blood.

What kind of blood you have is determined by the genes you inherit from your parents and is divided into different types, known as blood groups. The four main blood groups are A, B, AB and O.

Each group can be either RhD positive or RhD negative, which means that your blood group can be one of eight types (see below).

What is blood?

Your body carries around 4-6 litres (8.5-12.5 pints) of blood. Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma.

Plasma is about 90% water, but also contains proteins, nutrients, hormones and waste products. Blood is made up of about 60% plasma and 40% blood cells.

Each type of blood cell has a specific role to play:

  • red blood cells transport oxygen around the body and remove carbon dioxide and other waste products; they give your blood its red colour
  • white blood cells are part of the immune system (the body's natural defence mechanism) and help fight infection 
  • platelets help the blood to clot (thicken) to stop bleeding 

Blood groups

Your blood group is identified by antigens and antibodies present in the blood. Antigens and antibodies are your blood's natural defences against foreign substances.

Antigens are protein molecules found on the surface of red blood cells. Antibodies are found in plasma. Antibodies recognise anything foreign in your body and alert your immune system so that it can destroy it.

The ABO system

Blood groups are defined by the ABO system.

  • blood group A has A antigens on the red blood cells with anti-B antibodies in the plasma
  • blood group B has B antigens with anti-A antibodies in the plasma
  • blood group O has no antigens but both anti-A and anti-B antibodies in the plasma
  • blood group AB has both A and B antigens but no antibodies

Group O is the most common blood group in the UK, with 44% of the population having group O blood (37% O+ and 7% O-).

Receiving blood from the wrong ABO group could be life threatening because antibodies in a person with group A blood will attack group B antigens and vice versa. This will cause a severe reaction in the person receiving the blood.

As group O red blood cells do not have any A or B antigens, it can safely be given to any other group.

The NHS Blood and Transplant website has more information about the different blood groups.

The Rh system

Red blood cells sometimes have another antigen, a protein known as the RhD antigen. If this is present, your blood group is RhD positive. If it is absent, your blood group is RhD negative. This means that you can be one of eight blood groups:

  • A RhD positive (A+)
  • A RhD negative (A-)
  • B RhD positive (B+)
  • B RhD negative (B-)
  • O RhD positive (O+)
  • O RhD negative (O-)
  • AB RhD positive (AB+)
  • AB RhD negative (AB-)

About 85% of the UK population is RhD positive.

In most cases, O RhD negative blood can safely be given to anyone. However, this depends on specific antibodies and antigens being present in the blood.

Read more about what blood is used for.

Blood group test

To work out your blood group, your red cells are mixed with different antibody solutions. If, for example, the solution contains anti-B antibodies and you have B antigens on your cells, it will clump together.

If the blood does not react to any of the anti-A or anti-B antibodies, it is blood group O. A series of tests with different types of antibody can be used to identify your blood group, including groups other than the main ABO and RhD groups.

If you have a blood transfusion (where blood is taken from one person and given to another), your blood will be tested against a panel of donor cells that contain all of the clinically significant antigens. If there is no reaction, donor blood with the same ABO and RhD type can be used.


Pregnant women always have a blood group test. This is because if the mother is RhD negative but the child has inherited RhD positive blood from the father, it could cause complications if left untreated.

RhD negative women of child-bearing age should always only receive RhD negative blood.

Page last reviewed: 19/04/2013

Next review due: 19/04/2015


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The 7 comments posted are personal views. Any information they give has not been checked and may not be accurate.

girlieO said on 10 July 2014

Pls help! I got pregnant for my boyfriend and I had an abortion without him knowing about it, I just found out that he is O- and I'm A+. Pls can I still have children or do I need to have any kind of treatment?

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mariecov said on 21 August 2013

i also was told im rh negb in my first 3 pregnancies but my 4th child i was positive b i demanded new blood test which were given and still positive b but on all children i never had the anti d injection, i was told my blood group couldnt change but have proof off my file it has changed, can you explain how this occures. the day my 3rd child was born the blood test was positive b but was never told (first 3 children by 1 dad 4th child by another i dont know their blood groups)

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amy117 said on 25 January 2013

Can anyone help? Im AB negative and have 3 children. for the 3 I had an injection straight after the birth to do with this as I was negative I was told. My 4th pregnancy I miscarried and was told I didnt need an injection as I was now deemed to be ABpositive. I queried this but they stuck to their idea, which I was unhappy about and I miscarried my next baby.It worries me this lack of injection may have been the cause. needless to say the father of all my babies was the same. Can anyone put my mind at rest? Thanks

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D Olver said on 02 December 2012

The statement "This means that group O red cells can safely be given to anyone." under the heading Blood Group O, is in fact incorrect.

Only type O Rh D NEGATIVE blood can be safely given to any recipient. For instance, if anyone with O- A- B- or AB- received O Rh D POSITIVE Blood, it could potentially kill them.

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User701408 said on 31 July 2012

Will A+ mother carrying A- baby react to the baby?

And does this mean all human beings have the RhD factor in our blood group?

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sacrame said on 11 July 2012

is it safe for a female blood 0. neg to mate with male ABposs.

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Polly24 said on 27 February 2010

I am rh negative A

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