eli5: What happens when you get a blood transfusion with the wrong/incompatible blood type?
In: Biology
Your immune system treats the blood as foreign matters and try to kill it off. You will go into shock, cardiac arrest and die if not treated in time, and might still die even if treated in time.
Your body has antibodies that attack blood that isn’t the same kind as yours.
These immune cells physically rip apart the foreign blood cells.
This causes a few problems:
-The broken blood cells can cause a clotting reaction in the blood, and you blood goes from liquid to jelly
-Your body now has a ton of destroyed red blood cells that need to be broken down and gotten rid of. If not done fast enough the extra can be deposited in the skin and brain, which can cause damage
-Some people have such a big immune reaction they can go into anaphylaxis.
-You are dealing with the above issues, while already not healthy.
Every cell in the human body has proteins on the surface, some of which are used by the immune system to identify “friend or foe” for the cell. The human blood type is made of a set of the ones used by the immune system. As such, an incompatible blood type will result in an immune response, where the immune system decides the blood is an invader and begins attacking it.
The actual result will depend on the amount of incompatible blood transfused. With very small qualities, nothing too bad will happen, but the transfusion will be useless. The breaking down of the blood cells, however, increases the risk of blood clotting and releases various toxins into your blood. This can result in severe blood clotting, stroke, uncontrolled bleeding, and renal failure. These problems can be treated if caught quickly enough, though they are very dangerous and blood transfusions are typically given to already weakened patients.
It is worth noting that the common ABO/Rh blood system (where we get types like O+ and AB-) most people know is incomplete. Human blood has [51 different](https://en.wikipedia.org/wiki/Human_blood_group_systems) *known* blood type components, though many are fairly irrelevant*. Due to this, doctors will perform a final test with a small sample of the patient’s and donors blood, mixing them together (called [crossmatching](https://en.wikipedia.org/wiki/Cross-matching)) and checking for a reaction. Only if no reaction can be detected after a short time, will they actually give the transfusion. In emergencies, the transfusion may be started before this test is complete, though typically blood already tested for maximum compatibility will be used.
* Some of these other ones just don’t matter medically. Others are merely super rare. Also, note that I included A and B as two separate components, though they are normally treated as a single system – O is merely the lack of both A and B.
Your body begins to attack the foreign blood. Odds are if you had a blood transfusion, you needed the extra blood and your body destroying is both your body putting in extra effort and losing all that blood that just got added to you.
If you have type A blood, your blood has type A protiens on the blood cells. If you donate your blood to someone else, their body needs to already have those type A protiens, or else it sees a new protien is hasn’t seen before and attacks it.
It’s the same deal for type B, just replace protien A with protien B
Type AB blood has both A and B protiens on the blood cells. This is why an AB blood type can receive blood from anyone because their bodies are already familiar with both the A and B protiens, and knows not to attack it.
Type O blood has neither the A or B protiens, which is why type O blood can go to anyone, there’s no foreign protiens to detect. Type O people also can only receive other type O blood.
The Rh factor (positive or negative) is a different protien independent from the A, B, O blood type. There’s another protien, and if it’s positive, you have this Rh protien, and if it’s negative, you don’t.
An Rh negative person can donate the Rh positive person because there’s no new foreign protein for the immune system to attack. And Rh positive person can’t donate to an Rh negative person because the Rh positive blood will introduce a new protien that the recipient’s body will attack.
Putting all this together, O- can donate to everyone (universal donor). O+ can donate to anyone with an Rh positive blood type.
AB+ can receive blood from anyone (universal recipient) and AB- can receive blood from anyone with an Rh negative blood type.
A- can go to anyone with A or AB. A+ can only go to people with A+ and AB+
B- can go to anyone with B or AB, B+ can only go to people with B+ and AB+
AB- can only go to people with AB, and AB+ can only go to other AB+
There is one additional very rare blood type called Hh, or the Bombay blood type. These people can only donate blood to each other. There’s a missing antigen that the ABO system is built on.
Depending on how much: you die.
And not only that, but people who have had this happen report feeling a “[sense of impending doom](https://www.medicaldaily.com/tranfusion-reaction-mismatched-blood-type-donations-create-sense-impending-397528)” before they die.
Edit to add that this is why AB+ is the “best” blood type because it can take every single other blood type with not a single issue. It’s the opposite of O- and known as the universal receiver.
Your immune system can attack the new blood cells. This can cause a serious reaction called a hemolytic transfusion reaction. Symptoms might include fever, chills, back pain, and dark urine. In severe cases, it can lead to kidney failure or even be life-threatening.
Depending on the blood types involved, either your blood attacks the transfusion or the transfusion attacks your blood. At best, this results in you losing even more blood cells than you started with, and more often, since the blood is mixed throughout your body in seconds, the inflammation and cell damage causes cardiac and respiratory distress, multi-system failure, or even death.
Your blood cells has these little markers and so your body knows these markers are yours. These markers are the blood types. When a new marker blood enters(incompatible blood type) your body thinks its a foreign body and clots around it, think of this as an accident on highway. These clots block your blood highways (blood vessels) which means some parts of blood will not receive blood. Now blood is a delivery van for oxygen and when these body parts dont recieve blood they DIE. If a small part of your brain dies then YOU die.
I dont think it can be more simplified than this while telling the whole story.
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Assuming the blood types are incompatible (and not just different), your body’s immune system will treat the foreign blood as what it is: foreign. So it will attack it violently. If the foreign blood contained white blood cells, they might also attack your own blood. Which is not very good for your health.
Of note is that there are many more blood parameters to take into account to avoid adverse reactions. Blood type and rhesus are only the main ones.
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