Blood Typing

Blood typing is a classification system that allows healthcare providers to determine whether your blood is compatible with someone else’s blood. This is vital information for both blood transfusions and organ transplants as using incompatible blood can cause a severe immune reaction or even death.

There are 4 main blood types – A, B, AB and O. Each type can be either RhD positive or RhD negative, which means in total there are 8 blood groups.

Type O is the most common blood type in the UK (44%), while about 85% of the population is RhD positive.

How Are Blood Groups Inherited?

You inherit your blood type from your biological parents. The information for your blood type is situated on a single locus, called the ABO locus because the alleles of this locus have three distinct forms: A, B and O

You inherit one allele from each parent and the combination of your two alleles determines your blood type. The O allele is recessive, meaning that both of your inherited alleles will have to be O in order for you to express the blood type O. The A and B alleles are dominant.

When a dominant and a recessive allele are combined, the dominant allele is expressed. This means that, if you inherit an A allele from your one parent and an O from the other, your blood group will be A. The same is true for the B allele when combined with O.

Should you inherit both an A and a B allele, your blood group will be AB.

What Are Antibodies And Antigens?

Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma. Your blood group is identified by antibodies and antigens in the blood.

Antibodies are proteins found in plasma. They form part of your body’s natural defenses, – your immune system. They recognise foreign substances, such as bacteria and viruses, and alert your immune system, which destroys them.

Blood group antigens are either sugars or proteins that are attached to the red blood cell membrane. As some antigens can trigger a patient’s immune system to attack the transfused blood, safe blood transfusions depend on careful blood typing and cross-matching.

Receiving blood from the wrong ABO group can be life-threatening. For example, if someone with group B blood is given group A blood, their anti-A antibodies will attack the group A cells.

The ABO & Rh Systems

There are 4 main blood groups defined by the ABO system. These are:

  • 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

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 you can be 1 of 8 blood groups:

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

Which Blood Types Are Compatible?

It is essential for healthcare providers, especially in an emergency, to know your blood type. This is because your blood determines which blood types are safe for you to receive (usually in a blood transfusion). It also identifies which recipients can safely receive blood that you donate.

  • A positive – you can receive blood that’s A positive, A negative, O positive or O negative.
  • A negative – you can receive blood that’s A negative or O negative.
  • B positive – you can receive blood that’s B positive, B negative, O positive or O negative.
  • B negative – you can receive blood that’s B negative or O negative.
  • AB positive – you can receive any blood type.
  • AB negative – you can receive blood that’s AB negative, A negative, B negative or O negative.
  • O positive – you can receive blood that’s O positive or O negative.
  • O negative – you can only receive blood that’s O negative.

Universal Donor & Universal Recipient

People with blood type O negative (O-) are called universal donors. Anyone (with any blood type) can safely receive their blood. Type O negative blood has no antigens and will, therefore, not trigger an immune response, even if the recipient has a different blood type. This is why hospitals use type O negative blood the most in emergencies when someone needs blood fast.

Blood type AB positive (AB+) is the universal recipient. If you have this blood type, you can safely receive blood from any other blood type. Your blood recognises all potential antigens as safe, and your immune system will not launch an attack.

Pregnancy And Rhesus Disease

Rhesus disease is a condition where antibodies in a pregnant woman’s blood destroy her baby’s blood cells. It is also known as haemolytic disease of the foetus and newborn (HDFN). Rhesus disease will not harm the mother, but it can cause the baby to become anaemic and develop jaundice.

Rhesus disease can only occur if the mother has rhesus-negative blood and the foetus in her uterus has rhesus-positive blood. The mother must have also been previously sensitised to RhD-positive blood, usually during a previous pregnancy with an RhD positive baby. If the woman is exposed to RhD positive blood again, as with another pregnancy with an RhD positive baby, her body will immediately produce antibodies. These antibodies can cross the placenta, causing rhesus disease in the unborn baby.

To prevent this condition from occurring, pregnant women should always be given 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 can cause Rhesus disease.

Therefore, pregnant RhD-negative women, or any women who could possibly be pregnant, should only receive RhD-negative blood.

Frequently Asked Questions

AB negative is the rarest of the eight main blood types, with just 1% of donors having it.

If you are feeling ill or have a fever, you should wait until you feel better before donating blood. You should not have an infection at the time of donating and should be fully recovered for at least 2 weeks before donating your blood.

To donate blood, you should be aged between 17 and 65, and weigh between 50kg and 158kg.

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