The components of blood are:

• Red blood cells
• White blood cells
• Platelets 
• Plasma 

Each component makes up a different proportion of the blood. Red blood cells make up around 45%, white blood cells and platelets make up around 1%, and plasma makes up around 55% of the blood.

Red blood cells

Red blood cells are also known as erythrocytes. These are produced in the bone marrow. They carry oxygen and nutrients around the body. Red blood cells contain a protein called haemoglobin. Haemoglobin helps to carry oxygen from the lungs to the rest of the body. This is what makes blood the colour red. Carbon dioxide is then returned from the body to the lungs to be exhaled. Red blood cells have proteins on the cell membrane, these are called antigens. These antigens are markers on the cells, which allow them to be identified by white blood cells. The white blood cells are able to identify the red blood cells as being part of the body’s cells.

White blood cells

White blood cells are known as leukocytes. These are nucleated cells that fight infections to keep us healthy, so when you get sick it is the production of white blood cells that helps you to fight the cause of your illness. If white blood cells identify any other antigens on the cells that aren’t recognised as being the body’s cells, then the white blood cells will produce antibodies. These antibodies attach to the foreign and will destroy them.

Platelets

Platelets are also known as thrombocytes. These are fragments that help to stop bleeding and help to heal wounds by forming blood clots.

Plasma 

Plasma is the yellow liquid in which red blood cells, white blood cells, and platelets are carried. Blood plasma contains mainly water and other useful waste products. It contains proteins and hormones which help with the body’s functions. Plasma also helps to regulate bleeding and clotting.

There are 4 main blood groups:

• Type A
• Type B
• Type AB
• Type O 

Blood groups are identified by the antibodies and antigens in the blood. There are two types of antigens: antigen A and antigen B. The body has 3 alleles that code for a specific gene.

Allele coding:

Allele A codes for antigen A. These antigens are shown as purple circles in the image.

Allele B codes for antigen B. These antigens are shown as green diamonds in the image.

Allele O doesn’t code for either; therefore, there are no antigens shown.

Blood type A

Type A blood has A antigens, so antibodies in the body recognise these are being the body’s own cells. Type A blood has anti-B antibodies, so the antibodies will attack group B cells if they enter the body. This is because the antibodies can identify B antigens.

Blood type B 

Type B blood has B antigens, so antibodies in the body recognise these cells as being the body’s own. Type B blood has anti-A antibodies, so the antibodies will attack group A cells if they enter the body. This is because the antibodies can identify A antigens.

Blood group AB

Type AB blood has A and B antigens, and no antibodies.

Blood group O

Type O blood has no antigens, however, it has anti-A and anti-B antibodies. So white blood cells will attack A and B blood cells if they enter the body.

What determines our blood group?

Our blood group is determined by what we inherit from our parents, so we inherit a copy from both parents, this is where the 4 groups originate from. Blood type A and B alleles are dominant, and type O is recessive.

If you inherit AA or AO, then you will have type A blood.

If you inherit BB or BO, then you will have type B blood.

If you inherit AB, then you will have type AB blood.

If you inherit OO, then you will have type O blood.

Why is it important to know your blood type?

Knowing your blood group is important if you are receiving blood. People require blood all the time due to ongoing illnesses or in emergency cases where an individual loses too much blood. It can be life-threatening if an individual is given the wrong blood type.

An example is if someone with group A blood is given group B blood, then their anti-B antibodies will attack the group B cells. This can cause blood clots. Likewise, if someone with type B blood is given group A blood, then their anti-A antibodies will attack the group A cells.

Which type of blood can you receive?

• Blood type A can receive blood types A and O.
• Blood type B can receive blood types B and O.
• Blood type AB can receive blood types A, B, AB, and O.
• Blood type O can receive blood type O. 

Rh antigen 

Red blood cells can have another antigen known as the rh antigen, or sometimes called d antigen. This is inherited from parents. Rh genotype is dominant; therefore, if you are homozygous dominant (RR) or heterozygous (Rr), then you will be rh positive. If you are homozygous recessive (rr), then you will be rh negative.

Blood types:

• Type A+
• Type B+
• Type AB+
• Type O+
• Type A-
• Type B-
• Type AB-
• Type O-