system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When a person gets an organ transplant the body's immune system will reject and attack the organ because of foreign DNA. Why does this not apply to blood transfusions?
gpt
It \*does\* apply to blood transfusions. That's why you need to match the blood type. The blood type (A, B, AB, 0 etc.) refers to different kinds of proteins on the surface of the blood cells. If your body detects blood cells of the wrong type, it will attack and destroy them.

Blood type 0 does not have any of these proteins on the surface, so people of this type are "universal donors", i.e. their blood can be safely given to all other people regardless of blood type.

Conversely, people with blood type AB are "universal recipients". Because their own blood as both type A and type B proteins, their immune system won't attack blood of any type.

(Note that this isn't the full story, there are other blood type systems in addition to the AB system.)

Bonus info: With organ transfusions, one issue is that the recipient's immune system rejects the organ. But it sometimes happens that the immune cells from the donor that are still in the donated organ start attacking the recipients whole body (graft-vs-host disease)! This doesn't happen with blood transfusions because they typically filter out the white blood cells (immune cells) and only transfer the red blood cells and/or plasma.