All people belong to one of the main blood groups (types of blood): A, B, AB or O. Your blood group is identified by the antibodies and antigens contained in your blood.

This means, for example, that a person with the blood type O has no antigens, but both anti-A and anti-B antibodies in their blood plasma.

Similarly, a person with the blood type AB has both A and B antigens, but no antibodies in their the blood. Because the antigen and its antibody must not be mixed, blood transfusion can be safely performed according to a specific chart.

If the wrong type of blood is given to a person, the antibodies in their blood plasma react with the new red blood cells and cause them to stick together. This was the most common cause of death in blood transfusions before blood groups were discovered.

In addition to the ABO system, red blood cells sometimes have another antigen, a protein known as the RhD antigen.

Only RhD negative blood can be given to a Rhesus negative person. A Rhesus positive blood can be given to both blood types, as long as ABO portability is considered.

The blood that is given during a transfusion is tested to make sure it matches the blood of the recipient. Blood banks, where blood is obtained from voluntary donors and supplemented with anticoagulants, enable these transfusions. There is no harm in donating blood to a blood bank, as the body's blood volume will be restored within a couple of weeks. The donated blood is tested for diseases such as HIV. Blood is rarely transfused as such. Instead, red blood cells, platelets or blood plasma are often separated from the donated blood and given to people who need these specific parts of blood.