(USMLE topics) Cellular basis of blood groups including ABO, Rh (Rhesus) and other less known systems, why blood typing is important in blood transfusion.
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A blood type refers to the PRESENCE or ABSENCE of a certain marker, or ANTIGEN, on the surface of a person’s red blood cells. For example, in the ABO system, presence of A or B antigen gives type A or B, presence of both antigens gives type AB, while their ABSENCE gives type O.
Blood typing is critical for blood transfusion, as there are very SPECIFIC ways in which blood types must be MATCHED between the donor and recipient for a safe transfusion. The rule is simple: patients should NOT be given antigens that their own blood does NOT have. This is because the recipient’s immune system may recognize any “NEW” antigen as “FOREIGN” and develop antibodies to target it for destruction. Depending on the scale of the triggered immune response, the reaction can be serious or fatal.
Applying the rule, a type A patient, who is NEGATIVE for B antigen, can only receive blood from type A and type O donors, whose blood does NOT contain B antigen. A type AB patient, having both antigens, can receive blood from anyone, while a type O person, being NEGATIVE for both A and B, can only receive from type O donors, but can give blood to anyone.
Another important system is the Rh system, for which, D antigen, or Rh factor, is best known. The blood type for this antigen can be either Rh-positive or Rh-negative. By the same rule, a Rh-negative patient canNOT receive blood from a Rh-positive donor, while the reverse direction is fine.
Each of the 4 types of the ABO system can be Rh-positive or negative. This gives 8 possible combinations – the 8 basic blood types everyone knows about.
But ABO and Rh are only a FRACTION of the 35 currently known blood group systems, many of which can cause serious reactions during transfusion if mismatched. Altogether there are HUNDREDS of antigens, giving rise to a gigantic number of possible blood types. A fully specified blood type should describe the COMPLETE SET of antigens that a person has. In theory, this list must be determined for both donor and recipient before a transfusion can take place. In reality, however, most people only need to care about their ABO type and Rh factor.
The ABO and Rh systems are the most important in blood transfusion for 2 reasons. First, most people can produce ROBUST antibodies against A, B and D antigens, which may NOT be the case for other antigens. In fact, anti-A and anti-B antibodies are usually developed during the first year of life. Second, the 8 basic blood types are distributed in comparable proportions that make mismatching a likely event. Most other antigens occur at such frequencies that ONLY a VERY SMALL subset of patients is potentially at risk. For example, if 99.99% of a population is positive for a certain antigen and only 0.01% is negative, only that tiny fraction of negative patients is at risk regarding that antigen. To account for possible INcompatibility OUTSIDE ABO and Rh, an ADDITIONAL test is usually made before transfusion. A blood sample from the patient is mixed with a sample of donor blood and the mixture is examined for CLUMPS. No clumping means a compatible match.

In this video, Dr Mike explains the different ABO blood types and discusses who can give and receive blood in transfusions.

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