In a blood transfusion, does the donor's blood ever mount a reaction against the recipient?

Mae asks:

I would just like to ask a question about blood transfusion. Let's say that
the blood recipient is of the blood type "AB" and the blood donor is "O". 1 unit of Fresh Whole Blood "O" was transfused to the recipient. My question is: will the naturally formed antibodies (Antibody-A and Antibody-B) found in the donor's serum cause any transfusion reactions? *For sure, no sensitization will ever take place, but i'm just wondering about the antibodies present. For sure, Antibody A will bind to the A antigen. and so with the antibody B will bind with the recipient's B antigen.. So, will transfusion reactions take place?

That's actually a very good question - it shows that you're thinking critically.

We've all heard about transfusion reactions, and these are typically described as being caused when the recipient's blood cells attack the donor's blood. For instance, if I give AB blood to a patient who has blood group O, I am asking for trouble. The patient will have antibodies to both the A and the B antigens of the transfused red cells, and so his body will attack the blood. The reaction can be so strong that it causes collateral damage to his own body - and it sometimes leads to death. That's why we're so careful to try to match at least the ABO system and the Rhesus system (+ve or -ve) before we transfuse blood. (If you need a little background to blood groups, try here.)

But, as Mae points out, the opposite reaction could occur as well in certain circumstances. The example she uses is correct - if I transfuse O blood (which will contain anti-A and anti-B antibodies) into an AB patient, the lymphocytes in this blood will attack his body, won't they?

In fact, yes they do. But fortunately, this isn't usually a problem. Firstly, the blood we transfuse is usually "packed" red blood cells, with most of the white cells already removed from the mix. Secondly, and more importantly, the body's own lymphocytes will attack the foreign (donated) lymphocytes and quickly neutralise them. It tends to win this war since there are so much more white blood cells in the recipient's body than there are foreign ones in the small transfusion. So the effect is almost never clinically noticeable.

However, what if the host (i.e. recipient) didn't have enough of his own functioning lymphocytes? Well, then you would expect our worst fears to come true, and you'd be correct. Technically, the resulting attack, by the lymphocytes in the transfused blood, on the recipient's body is called graft-versus-host disease (GVHD), and is a risk whenever the recipient is severely immunosuppressed (e.g. chemotherapy, bone marrow failure, end-stage AIDS). It is also a risk when someone donates blood for one of their relatives, but that's another story...

So, Mae, the answer is that yes, the donor's blood can mount a reaction against the recipient's body. However, in a person with a normal immune system, this isn't a problem, since the troublesome foreign lymphocytes are quickly sorted out by the body's own immune system.