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Blood groups - Introduction

NHS Choices Medical Reference

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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.

Medical Review: April 17, 2013
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