Blood Grouping

Blood of human beings differs in certain aspects.

Blood transfusion is safe only when donor blood does not react with recipient blood. If RBC antigens meet matching antibodies, agglutination (clumping) happens, which can be dangerous.

Key Idea:

  • Antigen = present on RBC surface
  • Antibody = present in plasma
  • Same antigen + its antibody together = clumping (agglutination)

Two such groupings—the ABO and Rh—are widely used all over the world.

ABO Grouping

Basis of ABO grouping

ABO grouping depends on whether RBCs have:

  • Antigen A and/or
  • Antigen B

Plasma naturally contains antibodies:

  • anti-A (against antigen A)
  • anti-B (against antigen B)

Distribution of Antigens and Antibodies

Blood Group Antigens on RBCs Antibodies in Plasma Can Receive From
A A anti-B A, O
B B anti-A B, O
AB A and B Nil A, B, AB, O
O Nil anti-A and anti-B O

Key Rule to remember:

  • If you have antigen A → you will NOT have anti-A (body won’t attack self)
  • If you lack antigen A → you may have anti-A Same for B.

Universal Donor / Universal Recipient

  • Universal Donors (Group O): Persons with 'O' group blood have no antigens on their RBCs. They can donate blood to persons with any other blood group.
  • Universal Recipients (Group AB): Persons with 'AB' group blood have no antibodies in their plasma. They can accept blood from persons with ABO as well as other groups of blood.

Important Note: These “universal” terms are mainly for RBC transfusion. (In real transfusion practice, Rh and other minor groups are also checked.)

Rh Grouping

Another antigen, the Rh antigen (similar to one present in Rhesus monkeys), is also observed on the surface of RBCs of nearly 80% of humans.

  • Rh Positive (Rh+Rh^+): Individuals having the Rh antigen.
  • Rh Negative (Rh−Rh^-): Individuals devoid of the Rh antigen.

Rh Incompatibility (Erythroblastosis Foetalis):

Rh antibodies are NOT naturally present

Unlike ABO, an Rh⁻ person does not have anti-Rh antibodies naturally. They develop antibodies only after exposure (transfusion or pregnancy).

Rh Incompatibility: Erythroblastosis Foetalis (HDN)

A special case of Rh incompatibility (mismatching) is observed between the Rh−Rh^- blood of a pregnant mother and Rh+Rh^+ blood of the foetus.

  1. First Pregnancy: Rh antigens of the foetus do not get exposed to the Rh−Rh^- blood of the mother as the two bloods are well separated by the placenta. However, during delivery, there is a chance of exposure to small amounts of foetal blood. The mother starts preparing antibodies against Rh antigen.
  2. Subsequent Pregnancies: The Rh antibodies from the mother (Rh−Rh^-) can leak into the blood of the foetus (Rh+Rh^+) and destroy the foetal RBCs.
  3. Consequence: This can be fatal to the foetus or cause severe anaemia and jaundice. This condition is called Erythroblastosis Foetalis.
  4. Prevention: It can be avoided by administering anti-Rh antibodies (RhoGAM) to the mother immediately after the delivery of the first child.

Key Point:

  • Problem pregnancy = Rh⁻ mother + Rh⁺ baby (especially from 2nd pregnancy onward)
  • Prevention = RhoGAM after first delivery

Coagulation of Blood (Clotting)

Blood exhibits coagulation or clotting in response to an injury or trauma. This is a mechanism to prevent excessive loss of blood from the body.

Mechanism (Cascade Process):

  1. Thromboplastin Formation: Injured tissues and platelets release substances (Thromboplastins) which activate the coagulation mechanism.
  2. Prothrombinase Formation: A complex enzyme called Thrombokinase (or Prothrombinase) is formed by a series of linked enzymatic reactions.
  3. Thrombin Formation: This enzyme complex converts inactive Prothrombin (present in plasma) into active Thrombin. Prothrombin→ThrombokinaseThrombinProthrombin \xrightarrow{Thrombokinase} Thrombin
  4. Fibrin Formation: Thrombin, in turn, converts inactive Fibrinogen (soluble) into Fibrin (insoluble). Fibrinogen→ThrombinFibrinFibrinogen \xrightarrow{Thrombin} Fibrin
  5. Clot Formation: Fibrin forms a network of threads that trap dead and damaged formed elements of blood to form a clot or coagulum.

Role of Calcium: Calcium ions (Ca++Ca^{++}) play a very important role in clotting.

Blood Groups and Coagulation

  • ABO: Antigen on RBC, antibody in plasma → wrong match = agglutination
  • O: No A/B antigen → universal donor (ABO)
  • AB: No antibodies → universal recipient (ABO)
  • Rh: 80% are Rh⁺; Rh antibodies form after exposure
  • Erythroblastosis foetalis: Rh⁻ mother + Rh⁺ foetus (2nd pregnancy risk)
  • Prevention: RhoGAM after first delivery
  • Clotting chain: Thromboplastin → Prothrombinase → Thrombin → Fibrin → Clot
  • Ca²⁺ is essential

💡 Questions and Answers

Q1: Why is blood group O called the universal donor (ABO)?

Answer: Because O group RBCs don’t have A or B antigens, so they usually don’t get attacked by the receiver’s anti-A or anti-B antibodies. Key point: O = no antigen (A/B).

Q2: What happens if an Rh⁻ person receives Rh⁺ blood for the first time?

Answer: The person usually doesn’t show a big reaction immediately, but the body starts making anti-Rh antibodies. Next time Rh⁺ blood enters, it can cause a strong reaction. Key point: First exposure = sensitization.

Q3: Which enzyme converts fibrinogen into fibrin?

Answer: Thrombin converts fibrinogen (soluble) into fibrin (insoluble threads). Key point: Thrombin = fibrin maker.

Q4: Which ions are essential for blood clotting?

Answer: Calcium ions (Ca²⁺) are essential for proper clotting reactions. Key point: Ca²⁺ is mandatory.

Q5: Explain erythroblastosis foetalis in simple words.

Answer: If a mother is Rh⁻ and the baby is Rh⁺, the mother can make antibodies after the first delivery. In the next Rh⁺ pregnancy, those antibodies can cross placenta and break baby’s RBCs, causing anaemia and jaundice. Key point: Rh⁻ mother + Rh⁺ baby → 2nd pregnancy danger; prevent by RhoGAM.