What are blood types?

Blood is made up of four main parts:

• red blood cells,
• white blood cells,
• platelets, and
• plasma.

There are different types of blood because each red blood cell has special chemical markers called antigens. There are also antibodies (cells that fight particular groups of antigens) in the plasma of blood.

• Group A blood has A antigens on its red cells, and anti-B antibodies are contained in the plasma.
  
• Group B blood has B antigens on its red cells, and anti-A antibodies in the plasma.
  
• Group O blood has no antigens at all, but it has both anti-A and anti-B antibodies.
  
• Group AB blood has both A and B antigens, but no antibodies.

The Rhesus factor is also used to categorize blood as either Rhesus positive (+) or Rhesus negative (-). If blood is Rhesus positive, this means it contains the Rhesus antigen (and if it does not, the blood is Rhesus negative). Any blood type can be Rhesus positive or negative: for example, you can be A+ or A-.

Your blood type is inherited from your biological parents, in a similar way to eye or hair color. It is important to match blood groups when carrying out blood transfusions, so that the person's antibodies do not try to fight the new blood and vice versa.