bioquest said:
I've heard that people often die from blood transfusions
Is that because they've lost blood/had injuries? Is that why blood transfusions are so bad for the immune system?
It's not "often" that they die from blood transfusions, but mismatches can occur. People may die from blood loss in spite of a transfusion, if the blood can't be replaced quickly enough, or too much damage has already occurred before they got to the hospital for the transfusion.
If a mismatch in blood occurs, the immune system will react to the "foreign" substance (i.e., the transfused blood cells). When this happens throughout the entire body, because something like blood goes everywhere in the body, this would be severe and potentially lethal.
Blood from other species would not be compatible because proteins either composing the blood cells, or carried in the blood, would be somewhat different, and thus would be recognized as "foreign" when transfused.
The same considerations are made for organ transplants. There are other antigens on the surfaces of cells called Major Histcompatibility Complexes (MHCs) other than those used for simple blood typing. For a transplant, because you're not just temporarily replacing cells that will die and be replaced by the body in a short time, it's important to get more of them matched than is done for blood transfusions (the blood cells in the transfusion don't have to live very long). People receiving organ transplants are also put on very powerful immune suppressing drugs to further prevent the immune system from being activated by introduction of the "foreign" organ.
Something I've read about somewhat recently, but have not had a chance to look into in more depth, is that there may be some issues of using whole blood from women who have already had children for transfusions (other components can still be used, so no reason for women to stop donating blood). If I'm recalling that correctly, the issue was that there is just enough crossing of cells across the placenta (one of the reasons Rh factor is an important consideration during pregnancy) that antibodies are present in their blood that can attack the recipient even if they are type matched.
What one has to keep in mind is that someone who is being given a transfusion is getting it because they are already critically injured/ill and the risk of a complication from the transfusion is less than the risk of dying from the blood loss itself.