Blood groups and transfusions

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.