A discussion about irradiation and why we do it must first involve a quick blurb on Transfusion-associated Graft versus Host Disease (TA-GVHD for short).
Any time someone gets a transfusion of blood from someone who is not genetically identical to them (in other words, the donor and recipient have different HLA types), a fairly predictable series of events occurs. First, the transfused lymphocytes look around and say "Hey, this isn't me!" (not out loud, but you get the idea). As a result, they begin to do what lymphocytes do, which is to mount a cellular immune response against the host tissues. This could definitely be a problem if it weren't for what happens next. In someone with a normal immune system, the body responds to these foreign invaders with its own lymphocytes (primarily CD8 lymphs) and neutralizes them (let's call this the body's "counterattack"). This is illustrated in the top part of the figure below:
So, if a blood recipient is not capable of mounting this counterattack, the transfused lymphocytes can proliferate unchecked (see the bottom part of the above figure). This may not sound so bad, but the proliferation of foreign white cells can lead to significant damage. The process, as you have probably guessed, is called TA-GVHD, and you will see damage to the skin, gastrointestinal tract, liver, and, worst of all, to the bone marrow. In fact, the damage to the bone marrow is so significant and so permanent that TA-GVHD is said to be almost uniformly fatal. Your job, whether or not you choose to accept it, is to prevent this from happening!
Fortunately, we have discovered that irradiating lymphocytes before transfusion prevents them from mounting this attack on the host tissues that can cause so much damage. So, we flood blood components with irradiation in order to deactivate the transfused lymphocytes.
Obviously (at least I hope it's obvious), not everyone needs to have blood irradiated before transfusion. In general, this is for people who are not capable of mounting a counterattack and neutralizing transfused lymphocytes. People who fit into this category include stem cell transplant patients (and candidates), people undergoing chemotherapy, people with hematologic malignancies (especially Hodgkin's Disease), and people with congenital cellular immunity deficits. Term neonates are probably OK without irradiation, but many people choose to irradiate blood that goes to them anyway. The other category of people who need irradiated blood products is an odd one at first glance: Patients getting blood from a first-degree relative need irradiated blood, also. This is a topic for another FAQ.
Finally, you may be wondering about the effect of irradiation on the rest of the blood components. Irradiation actually does cause a bit of damage to red cells, as evidenced by the fact that a unit of red cells can only be stored a maximum of 28 days after they are "nuked." Other than that, though, the effects seem to be minimal.
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