Introduction:
Blood groups are specific classifications of blood based on the presence or absence of certain proteins (antigens) on the surface of red blood cells (RBCs). These antigens determine compatibility during blood transfusions, organ transplantation, and also have medicolegal importance in paternity testing.
The main blood group systems are:
- ABO Blood Group System
- Rh Factor System
- M and N Factors
(Other minor systems like P, Lutheran, and Lewis also exist but are less significant clinically.)
A. ABO BLOOD GROUP SYSTEM:
Concept of Agglutinogens and Agglutinins:
The ABO blood group system is based on the presence of agglutinogens (antigens) on RBCs and agglutinins (antibodies) in plasma.
- Agglutinogens A and B are the surface markers.
- Agglutinins α (anti-A) and β (anti-B) are present in plasma.
These determine four major blood groups: A, B, AB, and O.
| Blood Group | Antigen on RBC | Antibody in Plasma | Compatibility |
|---|---|---|---|
| A | A | Anti-B (β) | Can donate to A, AB |
| B | B | Anti-A (α) | Can donate to B, AB |
| AB | A and B | None | Universal recipient |
| O | None | Anti-A and Anti-B | Universal donor |
Note: These “universal donor” and “recipient” labels are only applicable within the ABO system. The Rh factor can change compatibility outcomes.
Formation and Characteristics of Agglutinogens:
- Agglutinogens start appearing by the 6th week of fetal life and reach adult levels by puberty.
- They are polysaccharide molecules found not just in red cells but also in organs like the liver, pancreas, salivary glands, and testes.
- Humans are divided into four main groups: A, B, AB, and O.
- Subgroups also exist: A1, A2, A1B, and A2B, with A1 and A1B being more common.
- The inheritance pattern follows Mendelian genetics with three allelic genes — A, B, and O.
B. Rh FACTOR SYSTEM:
What is Rh Factor?
The Rh factor (Rhesus factor) is another important antigen group discovered by Landsteiner and Wiener in 1940.
- About 85% of people are Rh positive (Rh⁺) — meaning their RBCs contain the D antigen.
- Those lacking this antigen are Rh negative (Rh⁻).
The Rh system includes six main antigens: C, c; D, d; E, e.
Among them, D is the most significant, as it strongly triggers antibody production.
Rh Incompatibility and Its Clinical Importance:
- Blood Transfusion Reaction:
- If Rh⁺ blood is transfused into an Rh⁻ person, the recipient’s immune system forms anti-Rh antibodies.
- On a second transfusion, these antibodies destroy donor RBCs, causing dangerous haemolytic reactions.
- Hence, Rh testing is essential before any transfusion.
- Erythroblastosis Fetalis (Haemolytic Disease of the Newborn):
- Occurs when an Rh⁻ mother carries an Rh⁺ fetus.
- The fetal Rh antigen can enter maternal circulation, causing her to produce anti-Rh antibodies.
- These antibodies cross the placenta and destroy fetal RBCs in later pregnancies.
- The baby may develop severe anemia, jaundice, or even die if untreated.
Prevention: Rh-negative mothers are given anti-D immunoglobulin (Rho(D) injection) soon after delivery or miscarriage to prevent antibody formation.
Rh Agglutinogens and Antibodies:
- Every person carries three Rh antigens (one from each pair: C/c, D/d, E/e).
- Dominant genes (C, D, E) make a person Rh⁺, while recessive genes (c, d, e) make them Rh⁻.
- The anti-D antibody is the most clinically important because it causes strong immune reactions.
There are two forms of anti-D antibodies:
- Complete (saline) antibodies: Act immediately in saline or albumin medium.
- Incomplete antibodies: Act only in special conditions (albumin or Coombs test).
C. M AND N FACTORS – MEDICOLEGAL SIGNIFICANCE:
The M and N blood group system is another inherited blood group identified by Landsteiner and Levine.
- It has three types: M, N, and MN.
- Though not important for blood transfusions, it is used in paternity testing and forensic studies.
Each person carries two blood genes:
- M + M → Group M
- N + N → Group N
- M + N → Group MN
If a baby’s blood group is M, both parents must carry at least one M gene. Similarly, MN combinations can help determine if a suspected person might or might not be the biological father.
The blood group substances are nitrogenous heteropolysaccharides containing D-galactose, fucose, glucosamine, and galactosamine.
D. P SYSTEM
Another minor group discovered by Landsteiner and Levine (1927) is the P system (P⁺ and P⁻). It’s not clinically significant but adds to the genetic understanding of blood classification.
Detailed Notes:
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