AssessmentsBlood groups and transfusions
Blood groups and transfusions
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 45-year-old man is evaluated in the surgical intensive care unit for a seizure. This is his first seizure episode. He was admitted to the hospital earlier in the day after being in a motor vehicle collision and losing a large amount of blood. Three units of packed red blood cells were transfused on arrival to the emergency department. Laboratory investigations show serum phosphate level within normal limits. Which of the following is the most likely cause of his seizure?
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.
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.
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.
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.
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.
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.
- "Medical Physiology" Elsevier (2016)
- "Physiology" Elsevier (2017)
- "Human Anatomy & Physiology" Pearson (2018)
- "Principles of Anatomy and Physiology" Wiley (2014)
- "International society of blood transfusion working party on red cell immunogenetics and terminology: report of the Seoul and London meetings" ISBT Science Series (2016)
- "B antigen acquired by normal A1 red cells exposed to a patient's serum" Transfusion (1987)