[Marika (OP)]

Garth wants to know:

It is said that only identical twins have the same DNA structure, I would like to know if one's DNA structure is compromised in any way if one receives either a blood transfusion, or if one receives an organ transplant from someone else?

What do you think?

[evan_au]

if one's DNA structure is compromised in any way..

If you have a blood transfusion or organ transplant, your body now contains DNA from a different person.
- By itself that does not change or degrade your own DNA
- Blood transfusions contain DNA only in the white blood cells, and these represent relatively little of the donated blood volume (mature red blood cells contain no DNA)
- Solid organs have a considerable amount of the donor DNA.

Careful matching is made of people who are donors and recipients, so the proteins expressed on the donor tissue are as similar as possible to the recipients.
- Despite this, organ recipients often have to be on long-term immune suppressant therapy.
- Without immune suppression, the donor may reject the transplanted organ, or the transplanted organ may reject the donor's body; either of these situations can kill you, which does degrade your DNA quite severely and permanently.
See: https://en.wikipedia.org/wiki/Graft-versus-host_disease
See: https://en.wikipedia.org/wiki/Tissue_typing

Sometimes microbiome transplants are done ("transpoosions"). This has been shown in animal studies to affect things like weight gain. So chemicals produced by bacteria in your gut can affect the expression of genes in your DNA.

[RD]

Blood transfusions contain DNA only in the white blood cells,
 and these represent relatively little of the donated blood volume (mature red blood cells contain no DNA)

On rare occasions donor white-cells survive and recipient becomes a chimera,
e.g. see ... https://www.ncbi.nlm.nih.gov/pubmed/17198844

[evan_au]

Another medical condition that can change your mix of DNA is pregnancy.
In perhaps half of pregnancies, cells from the fetus migrate across the placenta into the mother's circulation, where they can reach any tissue in the mother's body.

It is not clear if these fetal stem cells will help the mother (eg repairing damaged tissues) or harm the mother or child (eg setting up an immune response).

This is most easily detected when the baby is a boy - detecting male cells in the mother is a hint that microchimerism has occurred.
https://en.wikipedia.org/wiki/Microchimerism

Sometimes twins fuse in utero, resulting in a single baby with a mix of genes. This has resulted in a legal case where a woman bore a child which genetically appeared to be her niece or nephew.
https://en.wikipedia.org/wiki/Chimera_(genetics)#Humans