18.6 Blood Typing

Blood groups are essential for safe transfusions and preventing life-threatening reactions. The ABO and Rh systems determine compatibility, with each type having specific antigens and antibodies. Understanding these systems is crucial for healthcare professionals to ensure proper blood matching.

Incompatible transfusions can lead to severe consequences like agglutination, hemolysis, and shock. Knowing safe blood type matches and the genetics behind blood groups helps prevent complications in transfusions and pregnancies, especially for Rh-negative mothers carrying Rh-positive babies.

Blood Group Systems and Compatibility

Physiological effects of incompatible transfusions

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• Agglutination occurs when red blood cells clump together due to antibodies binding to antigens on incompatible RBCs (ABO mismatch)
  • Clumping can block small blood vessels and reduce blood flow to tissues leading to organ damage
• Hemolysis happens when red blood cells rupture due to antibody-mediated activation of the complement system
  • Hemoglobin released from ruptured RBCs can damage kidneys and lead to renal failure (acute tubular necrosis)
• Disseminated intravascular coagulation (DIC) involves widespread activation of the clotting cascade
  • Can cause both excessive clotting (thrombosis) and bleeding (hemorrhage) due to depletion of clotting factors
• Anaphylactic shock is a severe allergic reaction causing vasodilation, low blood pressure (hypotension), and difficulty breathing (dyspnea)
  • Can be life-threatening if not treated promptly with epinephrine and supportive care
• Transfusion reactions can range from mild to severe and may include fever, chills, and allergic reactions

ABO vs Rh blood group systems

• ABO blood group system
  • Four main blood types: A, B, AB, and O determined by the presence or absence of A and B antigens on RBC surface
  • Antibodies against missing antigens are naturally present in plasma
    • Type A has anti-B antibodies which attack B antigens
    • Type B has anti-A antibodies which attack A antigens
    • Type AB has no antibodies since both A and B antigens are present
    • Type O has both anti-A and anti-B antibodies since no A or B antigens are present
• Rh blood group system
  • Based on the presence (Rh+) or absence (Rh-) of the D antigen on RBC surface
  • Unlike ABO, anti-D antibodies are not naturally present and only develop after exposure to Rh+ blood (sensitization)
• Compatibility rules
  • ABO compatibility: Donor RBCs must not have antigens that react with recipient antibodies to avoid agglutination and hemolysis
    • Type O can donate to all types (universal donor) since their RBCs lack A and B antigens
    • Type AB can receive from all types (universal recipient) since they have no anti-A or anti-B antibodies
  • Rh compatibility: Rh- individuals should only receive Rh- blood to avoid developing anti-D antibodies which can cause problems in future transfusions or pregnancies

Safe blood type matches

• Type O-: Can donate to all types, but can only receive from O- to avoid both ABO and Rh incompatibility
• Type O+: Can donate to all Rh+ types (O+, A+, B+, AB+), but can only receive from O- and O+ to avoid Rh incompatibility
• Type A-: Can donate to A- and AB-, and can receive from A- and O- to avoid ABO and Rh incompatibility
• Type A+: Can donate to A+ and AB+, and can receive from A-, A+, O-, and O+ to avoid ABO incompatibility
• Type B-: Can donate to B- and AB-, and can receive from B- and O- to avoid ABO and Rh incompatibility
• Type B+: Can donate to B+ and AB+, and can receive from B-, B+, O-, and O+ to avoid ABO incompatibility
• Type AB-: Can donate to AB- only, but can receive from all Rh- types (O-, A-, B-, AB-) making them the universal Rh- recipient
• Type AB+: Can donate to AB+ only, but can receive from all types (universal recipient) since they have no ABO or Rh antibodies

Hemolytic disease in newborns

• Causes
  • Occurs when an Rh- mother carries an Rh+ fetus and fetal RBCs enter maternal circulation (fetomaternal hemorrhage)
  • During pregnancy or childbirth, fetal Rh+ RBCs can enter the mother's circulation and sensitize her immune system
  • The mother's immune system recognizes the D antigen as foreign and produces anti-D antibodies (alloimmunization)
  • In subsequent Rh+ pregnancies, maternal anti-D antibodies can cross the placenta and attack fetal RBCs causing hemolysis
• Consequences
  • Fetal RBCs are destroyed, leading to anemia (low RBC count) and jaundice (hyperbilirubinemia) in the newborn
  • Severe cases can result in hydrops fetalis (fluid accumulation in fetal tissues) and fetal death (stillbirth)
  • Treatment may include intrauterine blood transfusions (IUT) and early delivery to prevent further hemolysis
• Prevention
  • Rh- mothers are given Rho(D) immune globulin (RhoGAM) during pregnancy (28 weeks) and after childbirth
  • RhoGAM contains anti-D antibodies that neutralize any fetal Rh+ RBCs in the mother's circulation before her immune system can respond
  • This prevents the mother from producing her own anti-D antibodies (sensitization), protecting future Rh+ pregnancies from hemolytic disease

Blood Group Genetics and Inheritance

• Blood type inheritance follows Mendelian genetics principles
• ABO blood group is determined by three alleles: A, B, and O
  • A and B are codominant, while O is recessive to both A and B
• Rh factor is determined by a single gene with two alleles: D (dominant) and d (recessive)
• Blood group genetics influence antibody production against non-self antigens
• Blood banking practices rely on understanding genetic inheritance patterns to predict and manage blood type distributions in populations