Why do we Irradiate Blood Products?
I hope that the above BBGuy Podlet is helpful in explaining the rationale behind irradiation of cellular blood products! A summary of some of the info above follows:
Any 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 T-lymphocytes look around, see foreign HLA antigens, and say "Hey, this isn't me!" (OK, not out loud, but you get the idea). As a result, they begin to do what T-lymphocytes do, which is to recruit other T-lymphocytes to mount a cellular immune response against the host tissues (this interaction is very complex, and it involves cytokines and antigen presenting cells and a host of stuff that we don't need to talk about!). These T-lymphocytes especially target the patient's skin, gastrointestinal mucosa, liver, and bone marrow, though the bone marrow is potentially the biggest problem, as we will see below.
This T-lymphocyte "attack" on the recipient ("host") tissues would potentially 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 T-lymphocytes (primarily CD8 lymphs) and neutralizes them (let's call this the body's "counterattack"). This is illustrated in the figure below:
So, if a blood recipient is not capable of mounting this counterattack or for whatever reason chooses not to counter-attack, the transfused T-lymphocytes can proliferate unchecked (see figure below). This may not sound so bad, but the proliferation of foreign white cells can lead to significant damage. In fact, the damage to the bone marrow is so significant and so permanent that TA-GVHD is said to be almost uniformly fatal as a result of overwhelming infection. Your job, whether or not you choose to accept it, is to prevent this from happening!
Fortunately, we have discovered that irradiating T-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. The video above has more details on this process, including options for how to do it and the appropriate dosage to use.
Fortunately, not everyone needs to receive irradiated blood. In general, this is for people who are not capable of mounting a counterattack and neutralizing transfused lymphocytes. Patients who fit into this category include:
- Patients with congenital or acquired T-cell immune deficits
- Stem cell and marrow transplant recipients (and candidates)
- Patients with Hodgkin's Lymphoma (and probably other hematologic malignancies, if the patient is being treated with aggressive therapy)
- Babies getting transfused in-utero
- Premature babies that weigh less than 1200 g at birth
- Patients taking certain chemotherapy agents such as fludarabine that inhibit cellular immunity
- Granulocyte transfusion recipients
- Patients receiving blood from a first-degree relative (click for more detail)
- Patients receiving HLA-matched blood products
There is less uniform agreement about irradiated blood for certain other patients, such as those undergoing intense chemotherapy for non-hematologic malignancies,solid organ transplant recipients, and babies born at full-term (unless they received intrauterine transfusion)
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 and leads to low-grade hemolysis and a significant increase in extracellular potassium levels in irradiated red blood cells. As a result, in the United States, a unit of red cells can only be stored a maximum of 28 days after they are "nuked." The effect is minimal on platelets and on granulocytes, however, and irradiation does not change the expiration date of either product.
(Updated by DJC on 1/18/13)