Blood, blood components and blood types

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Learn about what blood does for the body, the different components of blood and what different blood types mean.

Key points

  • Blood carries oxygen all over the body and brings carbon dioxide to the lungs so it can be exhaled.
  • Blood contains red blood cells, white blood cells, platelets, and plasma.
  • There are four major blood types: A, B, AB, and O.
  • Different blood types cannot be mixed together.

What does blood do for the body?

Without blood, the body could not function. Blood carries the oxygen we breathe into our lungs to the parts of the body that need it. It also brings carbon dioxide back from the body to the lungs, so that it can be removed when we breathe out. Carbon dioxide is produced when the cells in the body make energy to power the body.

What are the different blood components?

Blood is made up of red blood cells, white blood cells, platelets, and plasma.

Red blood cells

Red blood cells are red because they contain a protein called haemoglobin. Oxygen and carbon dioxide attach to the iron in haemoglobin to be transported. The red blood cells carry oxygen from the lungs to the rest of the body and carry carbon dioxide from the body back to the lungs. Blood high in oxygen is bright red, while blood low in oxygen is blue.

Your body has millions of red blood cells. Each one lives for about 120 days. The bone marrow in the body is constantly making more red blood cells to refresh your blood, so you always have enough.

White blood cells

White blood cells fight off infection in the body. When they detect infection, they either attack it with antibodies or they surround bacteria and get rid of it.

White blood cells do not live as long as red blood cells. There are fewer white blood cells than red blood cells in the blood, although the body makes more white blood cells if it is about to fight an infection.


Platelets help blood clot to stop the bleeding from a cut or other injury.


Plasma is a clear liquid that makes up over half of the blood volume in the body. It carries the blood cells and platelets around the body. Plasma contains several important proteins and components to help fight off infections and to help with blood clotting.

What are the different blood types?

There are four major blood types: A, B, AB, and O. The genes we inherit from our parents determine our blood type. Different blood types cannot be mixed together, or the blood cells could start to clump together. When people have operations and need blood transfusions, the doctors need to know ahead of time what their blood type is so they can give them transfusions of the correct blood type.

Rhesus (Rh) factor

A person's red blood cells may also contain a protein called the Rhesus factor (Rh factor), which appears to help energize the body's cells. If you have the Rh factor, you are Rh positive, or Rh+. If you do not have the Rh factor, you are Rh negative, or Rh-.

Whether you have the Rhesus factor is indicated with your blood type. For example, if you have type A blood and are Rh positive, your blood type would be indicated as A positive, or A+. When children or adults need blood transfusions, they must receive blood that is Rhesus compatible.

People with this blood type......can donate to people with these blood types:
A+A+, AB+
A-A+, A-, AB+, AB-
B+B+, AB+
B-B+, B-, AB+, AB-
AB- AB+, AB-
O+A+, B+, AB+, O+
O-A+, A-, B+, B-, AB+, AB-, O+, O-

People with type A blood can donate to other people with type A, or people with type AB. People with type B can donate to other people with type B, or people with type AB.

AB blood can only be donated to someone else with AB blood. People with AB blood are considered to be the universal recipients (receivers), since they can safely receive any type of blood.

O- is known as the universal donor since people with any type of blood can safely be given O- as part of a transfusion. O+ blood may be given to any other Rh+ blood type.

People with type O blood can only receive blood donations from another type O blood donor.

Last updated: January 28th 2021