system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:how is blood transfusion biologically possible?

i don't know if i am being too stupid right now, but this question used to scratch my head for years. well, as i know due to my school-level biology knowledge, DNA's literally everywhere in one's organism; thus in a blood too.

what i can't get is the fact that blood transfusion is very viral and common in medicine nowadays; when one gets another human's blood, it's logical he gets different dna too. and what does happen then? are these two kinds of dna mixed together? how one can have dna different from his own and original one (and especially both his own and transfused ones?). 

shouldn't new blood change a person due to the fact he has new dna?
gpt
Actually, red blood cells, RBC for short, mature, their nucleus and its DNA shrinks and disappears.  This allows that concave shape that they appear to have, which fills with oxygen/CO2. Only mature cells have no DNA or nucleus.

The thing that gets tricky is the antigens and antibodies associated with the blood types. For example, if you have Type O blood, you hold no antigens (acceptable types of blood) to types A/B/AB, so your body will attack them because your body holds antibodies to these. (Keep in mind type O is recessive and considered harmless to the other types of blood)

Say you have Type A, you hold antigens to type A, meaning it's an acceptable type, but to O as well, since it's the recessive blood type. You have antibodies to B. The same way if you hold type B blood, just the opposite to A. If you have AB blood, you hold antigens for A, B, and AB (And O because O has no antigens since it's recessive and harmless)

As for the negative and positive, this is based off the plasma of the blood, being negative or positive. 

If the wrong blood is placed into the body, it causes an immune system attack, and you will most likely die. 

Source: school for hematology.