When an organ is replaced with someone else’s, the body needs immunosuppressants because of the foreign object now in the body. Why is the same not true for a blood transfusion?

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Basically title! I was wondering about this. Thanks!

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You might need some immunosuppressants with blood transfusions as well. However the blood that is being transfused is just temporary and does not linger long in the body. So there is little chance of rejection.

Blood cells don’t live very long (~120 days). The transfused ones live less time than “new” ones anyway, because of the time they lived in the donor. The idea is to keep you alive long enough for your body to make some new blood cells that match. The immune response kills a few along the way, it’s no big deal.

Transplant a kidney, and the concept is that it will continue to function for the rest of your life. That means it’s not OK for the immune system to kill it off a little at a time.

The blood doesn’t last long in the body which means that it doesn’t need to be as protected from your own immune system since your own body will be producing new blood all the time. On top of that most of what causes an immune system response is filtered out beforehand. Even when that’s not the case there’s not that much of it to begin with so it’ll only generally cause a mild reaction.

Organs on the other hand are full of things that cause an immune system response. On top of that they also contain parts of the donor’s immune system which means that also reacts against the new host. Since the “new” organ is supposed to last a long time immunosuppressants are needed to keep it viable for as long as possible since the host’s immune system will actively be trying to destroy it.

You don’t need immuno-suppressants because we understand how the immune system deals with blood: blood types. On the surface of your red blood cells, there are several different kinds of proteins, which proteins you have and which you don’t determine which blood type you are. Your white blood cells know which proteins you should have and will attack red blood cells with the wrong ones the same way it would attack any other foreign object.

In addition to what others said about blood not lasting very long in the body, there *is* an immune response against transfused blood, targeting certain surface molecules on blood cells. This is why blood types are so important for transfusions – blood type determines what surface molecules your blood cells have, and the immune system will attack anything that has a molecule you lack.

Source: I work in a blood bank at a hospital!

Some patients do! However, the type of immune response you get with blood transfusions are different than the kind you see with solid tissue just due to the difference in time and length of exposure and how easy it is to remove the offender from the body.

A semi-common transfusion reaction is actually hives/allergic reaction and some patients need to be pretreated with benadryl or antihistamines before transfusions.

Your immune system, specifically the adaptive immune system that is the main culprit in organ rejections, works by exposure and response. The body is exposure to a foreign something (like tissue from another person), it realizes that it’s foreign, and it starts to build an attack to it. This process takes time, although in some cases it happened quickly if the body has been exposure to something similar enough in the past that it basically can take a shortcut.

Blood transfusions are a single exposure to foreign stuff that don’t last long in your body. In fact, something like platelets which have a max lifespan of 7 days, so it’s gone from your body before your immune system has even figured out something weird happened. These are human cells, so it takes time for the body to realize that these human cells aren’t it’s human cells and for the immune system to go into attack mode. So most people don’t mount an immune response to blood transfusions whether platelets or red blood cells.

However, the immune system can! And does! For platelets, this normally shows up by decreased response to transfusion (basically the patient’s platelet count didn’t increase as much as it should). The body destroys the transfused platelets and removes them from the blood. This doesn’t normally cause any other major issues.

Red blood cells however are quite different. In order to receive crossmatched red blood cells (basically the standard for red blood cell transfusion that are nonemergency), you have to have blood bank testing. This testing includes a blood type and an antibody screen.

There are naturally occurring red blood cell antibodies. These are actually antibodies against sugars on bacteria you’ve been exposed to BUT these sugars can be found on red blood cells (or are similar enough) so they attack red blood cells too. However, the presence and absence of these antibodies are predictable based on blood type and can be detected by doing blood type testing. Group A blood types have anti-group B antibodies and part of providing someone is Group A is showing they have anti-B. So we know not give Group B blood to a Group A (that would trigger an immune response and be Very Bad).

There are non-naturally occuring red blood cell antibodies too. These are detected by the antibody screen and then identified. These are formed after the immune system has been exposed to red blood cells that are foreign either through transfusion or pregnancy. Some can also be trigger by other stuff, like exposure to a weird drug or bacterial that forms antibodies that have cross reactivity with red blood cells.

Antibodies are (mostly) specific, so they attack a specific aspect of the red blood cells that triggered the response. Through research, these immune system triggering aspects (aka antigens!) have been classified into various blood group systems (there’s a lot more than just the ABO blood groups you’re probably familiar with!). When a patient is found to have antibody, they get blood that DOESN’T have that aspect that triggers the immune system. We make sure the blood doesn’t have it by testing it. We also test the red blood cells that are going to be transfused against the patient’s plasma that contains the patient’s antibodies. If there is a reaction, then that blood is NOT given to the patient. If there is no reaction then the blood is as safe as we can prove and it’s assigned to the patient.

In an emergency situation, there may not be time to do testing before the patient bleeds out. So they get O Neg (or in some cases O Pos) because that is the safest blood type when considering just ABO groups. Most people have not been previously transfusion and therefore do not have non-naturally occuring antibodies, so the risk is minimum. But not zero.

If that patient did have antibodies, and the blood they were given reacted with those antibodies, they have a transfusion reaction. This reaction is their immune system destroying the red blood cells which causes all kinds of symptoms. At that point, you just treat the transfusion reaction and it’s symptoms – you can’t really do anything to stop it. Unlike a solid organ that remains in place and can continue to trigger the reaction thus making it worse and worse, blood is filtered by the body all the time and old, dying, or antibody covered cells are removed from the blood stream. So the trigger is naturally removed and there isn’t any long lasting presence that requires immunosuppressants.

Can you tell I like blood banking? Because I do.

We do have an immune response to tranfused blood. Cells of the immune system will ‘look’ at particular proteins on the surface of red blood cells and try to destroy those cells if those proteins are too different to those made by the host body.

But usually – as long as the correct blood type is used (it’s a way of classifying blood according to the type of proteins on cell surfaces) then it’s usually ok, and there’s no immune response, or it’s a very muted one that isn’t too catastrophic. It’s not uncommon for people to become a little feverish as the blood transfusion goes in: that’s the immune system having a bit of a grumble… it’s recognising that the incoming blood isn’t quite right, but it’s not sufficiently wrong to switch the immune response all the way on.

If someone is given blood of the wrong type, this can lead to a potentially very serious and potentially fatal immune response as the body goes all out to destroy the incoming blood cells, and this sets in chain a series of events that can be very harmful.

Sometimes people who have had lots of transfusions of blood or platelets can become quite sensitive to blood products, and transfusions make them feel quite ill. They may need ‘irradiated’ blood products, which have been treated with radiation to destroy any identifying proteins.

The immune system recognizes blood cells as self and will not attack them.

The immune system recognizes blood cells as foreign because they contain antigens that differ from the patient’s own antigens.

The patient’s immune system is suppressed so that it will not attack the foreign blood cells.

The patient is given an antiserum that will attack the new blood cells.

Because we are very careful to transfuse you with blood that your own body will not attack too much.

Blood transfusions are often just there as a temporary suppliment.

Blood is a very volitile cell type; Red blood cells only last 120 days, white blood cells, only 3, and neither of these cells are fresh when you get a transfusion.

By the time you’d get have issues with rejection, your body would’ve already gotten rid of them.

Some patients with reduced immune system needs Irradiated blood bags (red blood cells/platelets).
The minimum dose of radiation for the blood bag is (here in Germany) at least 25 gray.

Due to the production of the blood bag, a few white blood cells can still permeate the filter (from the whole blood bag) into the final product.

If you transfuse this blood bag to an patient with a low or incompetent immune system a “graft-versus-host-disease” can occur.

Blood Bank Scientist here.

Because in all but the most dire of emergency situations, we crossmatch blood for compatibility which helps to minimize the chances that the recipient will have a clinically significant immune response. We take a sample of patient plasma which contains their antibodies, mix it with the cells we intend to donate, then we look for a reaction usually using a card with a special gel in it. The gel allows cells that don’t have antibodies stuck to them, to pass, while holding antibody bound cells in place. When we look at the gel if we see all the cells at the bottom of the well the blood is deemed compatible, if we see cells anywhere else in the gel, it’s incompatible and we start looking into why, which is a whole different topic that goes beyond ELI5.

WRSaunders mentioned in another comment that red cells don’t live very long, while this is true, this doesn’t prevent an immune response. It’s still critically important that the blood we give out is compatible, the wrong blood can kill you in a matter of hours. What this does mean however is that we don’t have to consider the long term ramifications of the immune system constantly attacking because of small incompatibilities, organs are crossmatched to a much more stringent standard than blood but getting a 100% match is impossible. This isn’t a problem when you only need the donation for a short time until your body recovers it’s own cells, but if you need the donation over the long term to live, you need to get those low level immune responses in check or they’ll slowly degrade that organ.