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Understanding the Different Blood Types

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A blood type is a classification of blood, established by both the presence and absence of antibodies and inherited antigenic substances; this means that your blood group is determined by the genes inherited from your parents.

There are four main blood types, with each group being either RhD positive or RhD negative, resulting in 8 blood groups in total. Here’s everything you need to know to understand the differences between blood types.

Antibodies and Antigens

Blood is made up of red blood cells, white blood cells and platelets, which are found in a liquid called plasma. Platelets are small, colourless cell fragments within the blood that prevent bleeding by forming clots.

Your blood group is identified by both the antibodies and the antigens in the blood – antibodies being the proteins found in plasma, and antigens being protein molecules found on the surface of the red blood cells. Antibodies are part of our body’s natural defences, alerting the immune system once they recognise foreign substances, such as germs, so that the immune system can destroy these substances.

The Four Main Blood Groups

There are 4 main blood groups defined by the ABO system, and they are:

Blood group A – This group has A antigens on the red blood cells, with anti-B antibodies in the plasma.

Blood group B – This group has B antigens, with anti-A antibodies in the plasma.

Blood group O – This group has no antigens but has both anti-A and anti-B antibodies in the plasma.

Blood group AB – This group has both A and B antigens but is devoid of antibodies.

The most common blood group is blood group O, with one in seven people in the UK having blood group O blood.

It can be life-threatening to receive blood from the wrong ABO group – for instance, if a person with group B blood were to be given group A blood, their anti-A antibodies would attack the group A cells. For this reason, group A blood must never be given to somebody with group B blood, and vice versa. Blood group O, however, doesn't have any A or B antigens, and it can therefore be given to any other group safely.

The RhD System

Red blood cells can sometimes have another antigen, which is a protein known as the RhD antigen. If this antigen is present, the blood group is RhD positive, and if it’s absent, the blood group is RhD negative. As a result of this, you can be 1 of 8 of the following 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-)

Approximately 85% of the UK population is RhD positive, with 36% of the population being O+, which is the most common type. O- blood can, in most cases, be safely given to anyone, which is why it’s the blood type often used in emergencies when the blood group isn’t immediately known. The reason blood group O- is safe for most people is because it doesn’t have any A, B or RhD antigens on the surface of the cells, and is therefore compatible with every other ABO and RhD blood group.

Giving Blood

Most of us are able to give blood. However, currently, only 1 in 25 people actually do. You can donate blood if:

  • You’re fit and healthy
  • You’re aged between 17 and 66 years old (or 70 if you’ve given blood before)
  • You weigh at least 50kg (7st 12lb)
  • You are over 70 and have given blood in the last 2 years.

Testing Blood Groups

To identify your blood group, your red cells are mixed with different antibody solutions – for instance, if the solution contains anti-B antibodies and you have B antigens on your cells (you’re blood group B), it will clump together. If, however, the blood doesn’t react to any anti-A or anti-B antibodies, it’s blood group O. There are a series of tests involving different types of antibody that can be used to work out your blood group.

A blood transfusion, where blood is taken from one person and given to another, involves your blood being tested against a sample of donor cells that contain ABO and RhD antigens. If there is no reaction, then donor blood with the same ABO and RhD type can be used.


Women who are pregnant are always given a blood group test. The reason for this is if the mother is RhD-negative, but the child has inherited RhD-positive blood from the father, it could cause complications if left untreated. It’s important that RhD-negative women of child-bearing age always only receive RhD-negative blood.

Knowing your blood type is beneficial not only for your own health, but also in your ability to help others by donating blood. As you can see, there are various factors that can influence the blood type you have.