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Blood groups and blood types

You have 4-6 litres of blood pumping around your body, that's 7-10.5 pints.

Your blood group is determined by genes inherited from your parents.

Blood in the UK is categorised by the ABO system, and then whether it is RhD positive or negative.

A blood test is done to check which blood group a person is. A blood sample is taken from the arm and tested in a laboratory where it is mixed with antibody solutions. The way the sample reacts determines the blood type.

Knowing a person's blood group and blood type is important when giving blood or receiving a blood transfusion, as not all types are compatible with each other.

A routine test in pregnancy checks the mum's blood in case she has a certain blood type that's different to the one the child has inherited from the father. When this occurs problems can arise if treatment is not given.

Certain blood groups are more or less common depending on a person's ethnic background. NHS Blood and Transplant encourages more 'black Asian' and 'minority ethnic' donors to give blood to keep stocks of rarer blood types topped up.

ABO blood group system

The main blood groups are A, B, AB or O.

The groups get their names from protein molecules called antigens in the blood.

Blood group A has A antigens on the red cells, group B has B antigens, group AB has both A & B, while group O has neither.

Group O is the most common, with 48% of people in the UK falling into this category.

Rh blood system

Next is the Rh system, usually checking for the D antigen.

Around 85% of people have the D antigen, so are classed as D positive.

The 8 main blood groups are listed here, with the most common for UK blood donors first:

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

In a medical emergency where a person's blood group isn’t known, it is usually safe to use O RhD negative (O-) blood because it is compatible with every other blood group.

WebMD Medical Reference

Medically Reviewed by Dr Rob Hicks on May 15, 2017

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