Question of the Week Podcast

Question of the Week episode

Mon, 14th Jan 2013

Why aren't blood donations rejected?

Bags of blood collected during donation, showing dark colour of venous blood. (c) User "montuno" on Flickr

We find out why blood donations aren't rejected more readily. Plus we ask, is sleepiness related to cheerfulness?

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It's Jodie from Plymouth; my question is about donating blood.
If someone receives a donated organ, they have to be on drugs for the rest of their lives, so they don't reject the organ.
Why don't we have the same reaction when we receive donated blood?
You don't need to be related to the person; they can be a complete stranger, as long as they are the right blood group.

What do you think? evan_au, Thu, 27th Dec 2012

It is exposure to "novel" proteins that may trigger an immune reaction, and it takes a few days to weeks for a strong immune response to occur.

Blood transfusion differs from organ transplantation in several respects:


In trauma cases, the blood transfusion is intended to overcome a temporary shortage of red blood cells to carry oxygen. Red blood cells are regularly replaced by the body from the bone marrow, and so the patient does not rely on them in the long term.

A transplanted organ is a permanent replacement which must last a lifetime, so there is plenty of time for an immune response to develop, and any immune response will be potentially lethal. Long-term immuno-suppressants are needed in this case.

Number of novel proteins

Red blood cells have a fairly simple structure (they don't even have DNA). There are 3 main proteins which appear on the surface of red blood cells, and which frequently differ in a European population, producing the A, B and Rh groups; these are always considered during blood transfusion.

Other cell types have a greater variety of functions, and a greater variety of proteins appearing on the surface, which could potentially differ between donor and recipient. One such test mixes the White blood cells of donor and recipient, to see if they attack each other.   
For blood donations, you can almost certainly find a compatible donor for these 3 proteins from 100 random volunteers. For organ donation, you could easily check 100 people at random without a compatible match, so it's easier to start a donor search amongst the recipient's siblings, as they share many genes with the recipient.

Blood transfusion is similar to organ transplantation in several respects:

Organ transplantation always considers the ABO blood type, as this is another potential source of rejection
Blood type incompatibility can be severe in the case where a second foetus has a different blood type from the mother, as the mother has time to develop a strong immune response see

Transfusion of a young girl with the wrong blood type may later cause hemolytic disease when she becomes a mother

There are over 30 other proteins which may sometimes appear (or be missing) in different individuals or racial groups and can cause blood type incompatibility:
Individuals who receive frequent blood transfusions for chronic conditions are more likely to develop an immune reaction to some of these rare blood groups.
In theory, if everyone's DNA were on file, it would be possible to do a computerised search of the entire population to find potential donors with compatible MHC genes, even among non-relatives. But organ donation is a bit more permanent than blood donation.
evan_au, Thu, 27th Dec 2012

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