Blood groups and transfusions

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Blood groups and transfusions


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USMLE® Step 1 questions

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Blood groups and transfusions

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USMLE® Step 1 style questions USMLE

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A newborn is found to have jaundice that resolved in 48 hours. The mother’s blood group is O positive, and the fetus is A positive. The previous two pregnancies were also complicated by jaundice in the newborn, requiring phototherapy. Which of the following is the most likely cause of jaundice in this newborn?  

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Acute hemolytic transfusion reactions p. 112

Allergic reaction

blood transfusion p. 112

Anaphylaxis p. 110

blood transfusion p. 110

Anemias p. 425-417

blood transfusion therapy p. 436

Blood transfusions p. 436

reactions p. 112

Erythrocytes p. 415

transfusion of p. 436


transfusion reactions p. 112

Hypersensitivity reactions p. 110-111

blood transfusions p. 112

Immune responses p. 102-115

transfusion reactions p. 112


transfusion-related injury p. 112

Platelet disorders p. 434

transfusion for p. 435


complex concentrate transfusion p. 436

Pulmonary edema

transfusion-related injury p. 112

Thrombocytes (platelets) p. 415

transfusion of p. 429, 436

Thrombocytopenia p. 415

transfusion for p. 436

Type II hypersensitivity p. 110

blood transfusions p. 112


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.


Blood groups are determined by the presence or absence of certain antigens on the surface of red blood cells. The most important blood group system is the ABO system, which is determined by the presence or absence of two antigens: A and B. People with blood group A have antibodies against the antigen B, people with blood group B have antibodies against the antigen A. People with blood group AB have no antibodies to either antigen, and people with blood group O have both types of antibodies. There is also the D antigen which determines rhesus (Rh). People who have this antigen are said to be Rh positive, while those who do not are Rh negative. Blood transfusion refers to the process in which a person receives whole blood, or components of blood. A person with type O negative blood can donate red cells to any other blood type regardless of the rhesus, while a person with type O positive blood can donate red cells to any other ABO rhesus positive recipient. Those which have AB positive can receive blood from any other ABO regardless of the rhesus, whereas those with AB negative can receive blood from any other ABO with rhesus negative.


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "International society of blood transfusion working party on red cell immunogenetics and terminology: report of the Seoul and London meetings" ISBT Science Series (2016)
  6. "B antigen acquired by normal A1 red cells exposed to a patient's serum" Transfusion (1987)

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