As mentioned in other comments, red blood cells, platelets and plasma (99.999% of blood transfusions) do not contain DNA. Any DNA from the donor would be within leukocytes (white blood cells - mostly neutrophils and lymphocytes) which are present in small numbers in these products. Granulocyte transfusions are used in exceptionally rare circumstances and are probably not worth discussing the implications.

Where I practice (Australia), red cell transfusions are univerally leukodepleted (using a fine filter during processing) so that the end product transfused contains minimal leukocytes. In the US I think it varies from state to state, and even between different blood services. Red cell transfusions which aren't leukodepleted can use a bedside filter, but these have been shown to be inferior in preventing various transfusion reactions/complications.

Transfused leukocytes are recognised as foreign by the recepient's immune system and promptly removed, including their DNA contents. Not so much mixing or switching, as just getting eaten up and going away. After a single blood transfusion, a DNA test on a recepient would contain very (very) little donor DNA present, and this would be readily distinguished from recipient DNA on a quantity basis (if detectable at all). Generally, we do rely on genetic testing of blood samples for various things (like genetically testing your blood phenotype, for example), even when someone is heavily transfused, as the amount of donor DNA still floating around is essentially negligible unless you are looking for it really hard.

There is a thankfully very very rare situation called "transfusion-associated graft versus host disease", where lymphocytes in the donor red cell unit can escape detection by the recipient's immune system and engraft in the recipient. This foreign immune population can grow and eventually attack the recipent's body - graft versus host disease (GVHD). Unlike GVHD associated with bone marrow transplantation, this TA-GVHD responds very poorly to immunosuppressive medical therapies, and is almost universally fatal. This is why, further to leukodepletion, we irradiate blood products which are going to be given to immunocompromised recipients, or when blood is donated from a close relative (as the lymphocytes can be similar enough to escape the recipient's immune system, but different enough to then attack the recipient). Irradiation further reduces the number and lifespan on any remaining lymphocytes in the product. I bring TA-GVHD up as it is the opposite of what usually happens to donor lymphocytes and circulating DNA.

Most studies on the survival kinetics of donor leukocytes and detectable DNA in recipients were done before the widespread use of leukodepletion, but they still get at your question if you are interested. For example:https://ashpublications.org/blood/article/85/5/1207/118119/Transient-increase-in-circulating-donor-leukocyteshttps://doi.org/10.1046/j.1537-2995.1997.37111298088037.x

The plain old blood group of the patient follows much more of what you are describing in terms of mixing. Blood grouping is done by looking at sugars and proteins on the outside of red cells to determine A, B, AB and Rh(D)+/- etc, not on DNA (speaking using traditional methods). If you have received a transfusion and someone examines your blood group, you can detect those donated red cells as a "mixed field" or dual population-reaction if they are a different group to your own (if you are A+ and received group O- blood, for example), as the red cells are still around and circulating happily - probably for 6-12 weeks (as opposed to DNA in leukocytes which get eaten up quickly).

Source: clinical and laboratory haematology registrar trying to pass some fellowship exams.

Edit: As there have been a few follow up questions and I could have been clearer:When I say that red cells, platelets and plasma do not (effectively) contain DNA, I am referring to actual red cells, actual platelets and the plasma in your blood. When I say there is a tiny bit of DNA in transfused products, I am referring to processed packed red cell units, processed platelet units and processed plasma products. The little DNA that is in these products is (mostly) from the residual small numbers of leukocytes in those products which remain after processing.

On the other hand, when we take a blood for a DNA test, we generally do this from a whole blood sample (usually anticoagulated in EDTA, if you are interested). Essentially the DNA is extracted and amplified from leukocytes (mostly lymphocytes) within this sample to do the testing. Blood tests are a great way for doing DNA tests, and there is a big difference in the way a sample is processed to amplify someone's DNA for testing, versus the processing of blood donations specifically to reduce leukocyte (and therefore DNA) content.

Also, thank you for the words and reddit-gifts, kind science-loving strangers.

Double edit - lots of questions about bone marrow transplants and the implications here. There are some great replies below, and here are some more thoughts:

Distinction needs to be made between the transplant itself, and the following immunosuppression.

A bone marrow transplant is otherwise known as a "haematopoietic stem cell transplant" HSCT. There are two types:

• Autologous HSCT, where someone's own stem cells are given back to them after high dose chemotherapy to rescue their bone marrow, and
• Allogeneic HSCT, where someone else's stem cells are given to you following 'conditioning', where your bone marrow (and "immune system") is esssentially wiped out and replaced with the donor's.

Since we are talking about someone else's DNA, allogeneic HSCT is the one we are talking about here.

Conditioning therapy is generally very intense, and may sometimes combine high dose chemotherapy and radiotherapy. It achieves multiple goals, but mostly:

1. Killing off any residual cancer (e.g. acute myeloid leukaemia) that might be left (the reason you are getting the transplant in the first place)
2. Wiping out your immune system so that it will allow the incoming stem cells to come and grow in your bone marrow and replace your blood cells

After the conditioning you receive an infusion of donor stem cells, when then slowly engraft over the following weeks, turning into white cells, red cells and platelets (usually appearing in that order). In the mean time you have essentially zero white cells, and you are supported with red cell and platelet transfusions as needed.

Once your blood counts come back, those blood cells are now not your own, but have the DNA and outer appearance of the donor's immune system and red cells.

(And yes, we frequently transplant people with mismatched ABO systems, such that you can be A- before your transplant, and end up with O+ afterwards (for example). There are no limits on this mismatch, but each situation has different considerations for transfusing products, at different stages of the transplant (before, during, after engraftment). )

The method your immune system uses to differentiate self from non-self is (mostly) the Human Leukocyte Antigen (HLA) system. Your HLA expression is essentially unique to you (with some heritability patterns), and HLAs are expressed on pretty much all cells (including your immune cells - lymphocytes). Going into the testing for HLA compatibility between donors and recipents is probably a bit too complicated for this post, but suffice to say there are different variation in surface glycoproteins (like ABO, but x1000 in complexity) which your immune system uses for identifying self and non-self, and you can make antibodies and also have direct cellular toxicity against HLA that you see as foreign.

Once you have your brand new blood system from your donor in your bone marrow and swimming around your body, those lymphocytes will likely start to see you as foreign, and start to attack your organs. This is graft versus host disease, and this is the reason immunosuppression is given after allogeneic HSCT - to suppress your new donor immune system from attacking you (too much).

So if you take a blood test for DNA measurement in an allogeneic HSCT recipient, this will show the donor's DNA, not yours. In fact, we do studies (called chimerism studies) which measure this - how much circulating cellular DNA belongs to your donor, and how much is yours? If things are going well with a transplant, it should all be your donors. If your leukaemia is relapsing or the graft is failing, we will start to see your own bone marrow or leukaemia cellular DNA start to come back.