A blood transfusion is the procedure where a person receives blood, or elements of blood usually through an intravenous infusion - meaning through a vein.
Now - if you take blood and spin it in a centrifuge, the heaviest blood components move to the bottom, and the lightest ones move to the top.
Overall, three distinct layers form: the erythrocytes or red blood cells at the bottom, the buffy coat--which contains platelets and immune cells in the middle, and plasma at the top.
Now, in rare situations, like in traumatic injuries, someone might receive a whole blood transfusion, but more commonly a person is given one of the components of the blood.
For example, a person with anemia could just receive packed red blood cells, a person with clotting factor deficiency could get fresh frozen plasma which contains the coagulation factors, or someone with platelet deficiency might receive platelets.
Now, most blood transfusions are homologous transfusions, where the blood comes from an anonymous donor.
Sometimes the transfused blood is autologous, meaning the blood was taken out of the person at a prior time, like when they plan to have surgery in the near future.
In both cases, once the blood is taken, it’s mixed with sodium citrate which prevents the blood from coagulating, and then refrigerated or frozen for storage, or separated into its components by centrifuge.
Now, before whole blood or packed red blood cells can be transfused, it’s important to know the blood typing of both the donor and the recipient of the blood.
Every person has a unique blood group based on two classification systems: the ABO system and the Rh system.
Both systems are based on the presence or absence of glycoproteins, which are proteins attached to a sugar molecule, found on the surface of red blood cells.
Now, if blood that has any of these glycoproteins is given to a person that has immune cells that have never seen those glycoproteins before, then the glycoproteins can act as antigens.
In other words, the recipient’s immune system might mistake the donor blood as an invader and mount an immune response.
This is usually caused by recipient IgM antibodies that float around in the plasma. These IgM antibodies can bind to multiple donor red blood cells, which can result in the red blood cells clumping together.
In addition, the IgM can activate complement proteins which form a membrane attack complex on the surface of the donor red blood cell causing pores to form and allowing the red blood cell to lyse.
This is called a hemolytic transfusion reaction. The IgM also activate other immune cells to cause a systemic immune response resulting in fever, hypotension, and rash.
Now, the ABO system refers to the type of glycoproteins found on a person’s red blood cells; you can have either type A, type B, type A and B glycoproteins, or neither, which is called type O blood.
The immune system produces antibodies against the glycoproteins that you don’t have.
People with type A blood have antibodies to type B blood, and people with type B blood have antibodies to type A blood.
People with blood type AB don’t have antibodies to type A or type B blood, so they can receive blood from any of the blood types.
Thus, these lucky people are universal recipients. However since their red blood cells have both type A and type B antigens, they can’t donate blood to anyone except other AB blood type individuals.
So they’re good receivers but bad givers. On the other hand, people with type O blood have antibodies to both A and B glycoproteins, so people with type O cannot receive type A, type B, or type AB blood.