Why can't a patient receive blood from a family member?

 This is a great question! One of the better transfusion related questions in a long time, in fact. The short answer to your question is that the chance on “transfusion-associated graft versus host disease” (TA-GvHD) is higher when you receive blood from relatives. Explaining what this means and why this is the case is a little more complicated, but bear with me.

When you receive a blood transfusion from a random donor, what you typically receive are packed red blood cells. The blood plasma, platelets and white blood cells are removed as much as possible. However, some of these components still remain in the unit of packed cells. These components pose some risks to the patient and are therefore eliminated as much as possible. One of those risks comes from the donor’s white blood cells (their T-cells, to be exact).

T-cells are part of our bodies’ immune system. They recognise and attack cells that are foreign to our body. When you receive a blood transfusion. The T-cells of the donor therefore recognises your cells and tissue as foreign. When left unchecked, the T-cells proliferate (multiply) and will start to attack and destroy your body from the inside. This is called a “Graft versus host effect”: the graft (the blood of the donor) is attacking the host (the patient). When this happens with transfusions specifically, we call it a “Transfusion-associated graft versus host disease” (TA-GvHD).

Fortunately, the patient has T-cells as well. And those of the patient normally are far more numerous than those of the donor. A small battle will be fought inside the blood stream. When the patient has a healthy immune system, his T-cells will win this battle handily.

Figure. T-cells attacking a foreign cell.

However, when the immune system of the patient is compromised, for example due to chemotherapy, this battle may be won by the T-cells of the donor. When this happens, the patient will suffer from TA-GvHD. This is a very serious complication with a high mortality rate. There is no effective cure for it.

Finally, when we transfuse blood from a close relative (1st or second degree), the blood of the donor may be so similar to the blood of the patient that the patient’s immune system does not recognise the donor’s T-cells as foreign. In this case, there will be no battle. To the patient’s T-cells, those of the donor are friendly. However, the donor’s T-cells will still recognise the patient’s tissue as foreign and will proliferate and attack unimposed. This may lead to TA-GvHD even in patients with a healthy immune system.

The only way to eliminate this risk is to irradiate the packed cells before transfusion with 25 Gy Gamma radiation. This will completely destroy all of the donor’s T-cells. However, this option is expensive, requires specialised equipment and also damages the red blood cells resulting in a less effective transfusion. The best solution therefore, is to not transfuse blood of relative up to the 2nd degree unless absolutely nescessary.