COVID-19 Information - To see our up to date advice for using our pharmacies - Click Here

Health Knowledge and Encyclopedia

At your local Pearn's Pharmacy we can offer advice on most general health matters. You can also use our Health Encyclopaedia to provide you with the tools and links you need to pinpoint symptoms and get a full explanation of a suspected condition.

Search By Letter
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y |

Search by Keyword

Blood groups


People can have different blood types, known as blood groups. There are four main blood groups - A, B, AB and O. Each group can then be either RhD positive or negative, so your blood group can be one of eight types.

The genes you inherit from your mother and father determine your blood group.

What is blood?

Your body carries around four to six litres (8.5 to 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 made up of 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 of the three types of blood cells has specific roles 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 fight infection as part of the immune system (the body's natural defence against infection and illness). 
  • Platelets help the blood to clot (thicken). 

Blood groups

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

Antigens are protein molecules that are found on the surface of red blood cells and antibodies are found in the plasma. They recognise anything unusual 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 its red blood cells and anti-B antibodies in its plasma.
  • Blood group B has B antigens and anti-A antibodies in its plasma.
  • Blood group O has no antigens but both anti-A and anti-B antibodies. This means that group O red cells can safely be given to anyone. It is the most common blood group in the UK.
  • Group AB has both A and B antigens but no antibodies, otherwise it would destroy itself.

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.

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, you are 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-)

Around 85% of the UK population is RhD positive.

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 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 antibodies, it is blood group O. A series of tests with different types of antibody will identify your blood group.

If you have a blood transfusion, 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 Rh type can be used.


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

Giving blood

Most people are able to give blood but only 4% actually do.

If you are generally in good health, 17 to 65 years of age (if it's your first time) and weigh at least 7st 12lb (50kg) you can donate.

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

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

Find more information about donating in South Wales. or you can also call 0800 25 22 66.

^^ Back to top

What is it used for?

Donated blood is used to replace blood loss during surgery or an accident.

In the UK, blood is not used whole but is separated into red cells, platelets and plasma so that it can be used in various treatments. In cases where a severe amount of blood has been lost, all three blood components can be transfused.

Blood transfusions

blood transfusion is where blood is taken from one person and put into another person, directly through the blood vessels (intravenously). Some blood groups cannot be mixed with each other, so the blood group you receive must be compatible with your own.

For example, if you are blood group A, you cannot take red cells from a person with blood group B because the anti-B antibodies in your blood will fight the B antigens in the donor's blood. This causes the donated red cells to be destroyed, which can be fatal.

Blood group O- red cells have no ABO or RhD antigens and can be given to almost anyone. Someone with blood group AB+ can usually receive red cells from any group, but AB+ red cells can only be given to a person with blood group AB+.

Your blood group will be identified by a blood group test before a transfusion takes place.

How blood is used

Blood is separated into its various parts and each part is used to treat different conditions. There are many conditions that are treated by donated blood.

Red blood cells

Red blood cells are used to treat all kinds of anaemia, as well as cancer, rheumatoid arthritis and sickle cell disease. They can also be used to replace red blood cells that are lost in accidents, surgery and childbirth.


Platelets can be used to treat bone marrow failure and leukaemia (cancer of the white blood cells). They can also be used with chemotherapy and in patients who have had a transplant.


Plasma can either be frozen or processed for use in a wide variety of treatments.

Frozen plasma is used to replace blood that is lost during childbirth or following injury. It may also be needed in conditions that decrease the production of clotting factors, such as liver disease. A protein called albumin that is found in plasma can also be used to treat burns.

Processed plasma is important for the treatment of all types of haemophilia (a genetic condition that affects the blood's ability to clot).

It can also help treat pregnant women with RhD negative blood who are carrying a baby with RhD positive blood inherited from the father.

Processed plasma can also be used when producing antibodies that are used to treat conditions such as:

^^ Back to top
“Pearn's Pharmacies, putting patients first”

Pearn's Pharmacy Services

Common Ailment Service

Primary Choice is a campaign to help the public choose the right health advice in the community.

Tell Me More


Repeat, one-off prescriptions. Collect in store or home delivery.

Sign Up

Funded Services

Browse our NHS funded services offered in our stores.

See Services


We are receiving an increase in the number of requests for delivery of medicines due to the COVID-19 pandemic.

Important Info

Find a GP   

Use our GP locator service and nearest Pearn's Pharmacy Branch

Search Now