The blood was irradiated to prevent any problems the white blood cells in the blood transfusion might cause you. Rarely, people with leukaemia and lymphoma are at risk of damage to their bone marrow, gut, or skin caused by white blood cells in transfusions. This is called transfusion-associated graft versus host disease (TA-GvHD). And, irradiating blood prevents TA-GvHD in people who are at risk.
When blood is collected from a blood donor, some white cells are collected as well as the red cells. These white cells go into your blood during a transfusion. In most people the donor white blood cells are killed and removed by their immune system and so don't cause any problems. But, if you have a leukaemia or lymphoma your immune system may not work as well and so may not be able remove all of the donor white blood cells.
White blood cells are part of our immune system and one of their jobs is to destroy 'foreign' cells coming into the body. If your immune system isn't working properly there is a risk that the donor white cells will stay in your bloodstream. These transfused white cells see your body as ‘foreign’ and could multiply and attack it, leading to TA-GvHD.
Irradiating blood stops donor white cells from harming you but it doesn't damage red blood cells or platelets and doesn't make blood radioactive. So, a blood transfusion that has been irradiated can't harm you or anyone around you.
Not everyone who has leukaemia or lymphoma needs to have blood transfusions irradiated. It depends on the type of leukaemia or lymphoma they have and on the treatment they are given. Some people will always need to have their blood and platelet transfusions irradiated. Other people only need to have this done for a short time. Your doctors will be able to tell you how long this precaution needs to be taken for you.
If your doctors say that you will need blood transfusions to be irradiated for sometime you may be given a card to carry that explains this. And, until your doctors tell you it is safe for you to have non-irradiated blood you should always check that any blood or platelets you are given are irradiated.