Human Physiology

Human Blood Groups

1. Introduction

Human blood groups are classifications of blood based on the presence or absence of specific antigens on the surface of red blood cells (RBCs). These antigens are genetically determined and are important in:

Blood transfusion
Organ transplantation
Pregnancy and fetal health
Forensic science
Anthropology and genetics

The most important blood group systems for NEET are:

ABO Blood Group System
Rh (Rhesus) Blood Group System

2. Basic Terminology

Antigen (Agglutinogen)

An antigen is a substance present on RBC membrane that can stimulate immune response. Examples:

Antigen A
Antigen B
Rh antigen (D antigen)

Antibody (Agglutinin)

Antibodies are proteins present in plasma against absent antigens. Examples:

Anti-A antibody
Anti-B antibody

3. Discovery of Blood Groups

The ABO blood group system was discovered by Karl Landsteiner in 1900. He received the Nobel Prize in 1930.

4. ABO Blood Group System

The ABO system is based on:

Presence or absence of antigen A and antigen B on RBCs
Presence of antibodies in plasma

5. Types of ABO Blood Groups

Blood Group	Antigen on RBC	Antibody in Plasma
A	A	Anti-B
B	B	Anti-A
AB	A and B	None
O	None	Anti-A and Anti-B

6. Principle of Blood Grouping

When incompatible blood groups are mixed:

Antigen reacts with corresponding antibody
RBCs clump together
This clumping is called agglutination

Agglutination may cause:

Blockage of blood vessels
Kidney failure
Death

7. ABO Blood Group Reactions

Group A Blood

RBCs contain antigen A
Plasma contains anti-B antibodies

Can receive:

Cannot receive:

Group B Blood

RBCs contain antigen B
Plasma contains anti-A antibodies

Can receive:

Cannot receive:

Group AB Blood

RBCs contain both A and B antigens
Plasma has no antibodies

Can receive all blood groups:

Hence called:Universal Recipient

Group O Blood

No A or B antigens
Plasma contains both anti-A and anti-B antibodies

Can donate to all groups.

Hence called: Universal Donor

(Especially O negative in practical transfusion medicine)

8. Genetics of ABO Blood Groups

The ABO blood group is controlled by gene I.

There are three alleles:

9. Nature of Alleles

IA and IB

Dominant over i
Show codominance with each other

i allele

Recessive

10. Genotypes and Phenotypes

Blood Group	Genotype
A	IAIA or IAi
B	IBIB or IBi
AB	IAIB
O	ii

11. Codominance

In blood group AB:

Both IA and IB express equally
Both antigens appear on RBC surface

This phenomenon is called: Codominance

12. Multiple Alleles

Presence of more than two alternative forms of same gene in population is called: Multiple Allelism. Example:

13. Inheritance of ABO Blood Groups

Example

Parents:

IAi × IBi

Possible children:

Genotype	Blood Group
IAIB	AB
IAi	A
IBi	B
ii	O

Thus, all four blood groups are possible.

14. Bombay Phenotype (Important Concept)

Discovered in Bombay (Mumbai), India.

Individuals:

Genetically may possess IA or IB genes
But cannot produce H substance required for A/B antigen formation

Therefore:

Blood appears as O group
Cannot receive normal O blood

Very rare condition.

15. Rh Blood Group System

Discovered in rhesus monkeys by:

Karl Landsteiner
Alexander Wiener

16. Rh Antigen

The most important Rh antigen is:

D antigen

17. Rh Positive and Rh Negative

Type	D Antigen
Rh+	Present
Rh−	Absent

Most humans are Rh positive.

18. Importance of Rh Factor

Rh factor is important in:

Blood transfusion
Pregnancy

19. Rh Incompatibility

Occurs when:

Rh− person receives Rh+ blood

First transfusion:

Usually mild
Body develops anti-Rh antibodies

Second transfusion:

Severe agglutination and hemolysis may occur

20. Erythroblastosis Fetalis

(Hemolytic Disease of Newborn)

A very important NEET topic.

Cause

Occurs when:

Mother is Rh−
Fetus is Rh+

Usually:

Father is Rh+

21. Mechanism

First Pregnancy

Fetal RBCs may enter maternal blood during delivery
Mother develops anti-Rh antibodies
First child usually safe

Second Pregnancy

If second fetus is Rh+:

Maternal anti-Rh antibodies cross placenta
Destroy fetal RBCs

This causes:

Hemolytic anemia
Jaundice
Brain damage
Death of fetus

22. Prevention of Erythroblastosis Fetalis

Prevented by giving:

Anti-Rh antibodies (Anti-D injection)

to Rh− mother:

Within 72 hours after first delivery

This prevents formation of maternal antibodies.

23. Blood Transfusion

Definition

Transfer of blood from donor to recipient.

24. Blood Compatibility Chart

Recipient	Can Receive
A	A, O
B	B, O
AB	A, B, AB, O
O	O only

25. Universal Donor and Universal Recipient

Type	Importance
O−	Universal donor
AB+	Universal recipient

26. Cross Matching

Before transfusion:

Donor RBCs are mixed with recipient serum
Tested for agglutination

Purpose:

Prevent transfusion reactions

27. Blood Typing Procedure

Blood sample is mixed separately with:

Anti-A serum
Anti-B serum
Anti-D serum

Agglutination indicates presence of corresponding antigen.

28. Other Blood Group Systems

Besides ABO and Rh:

MN system
Kell system
Duffy system
Lewis system

29. Importance of Blood Groups

Medical Importance

1. Blood Transfusion

Prevents fatal reactions.

2. Organ Transplantation

Matching reduces rejection.

3. Pregnancy

Prevents Rh incompatibility.

4. Forensic Science

Used in crime investigation and paternity testing.

5. Anthropology

Helps study human evolution and migration.

30. Blood Donation Facts

Healthy adults can donate blood.
Average donation: about 350–450 mL
O negative blood is highly valuable in emergencies.

31. Agglutination Reaction

Antigen-antibody interaction causes:

RBC clumping
Hemolysis

This may lead to:

Shock
Renal failure
Death

32. Important Terms for NEET

Term	Meaning
Agglutinogen	Antigen on RBC
Agglutinin	Antibody in plasma
Agglutination	Clumping of RBCs
Hemolysis	Destruction of RBCs
Universal donor	O−
Universal recipient	AB+
Codominance	Equal expression of IA and IB
Multiple alleles	More than two alleles in population

36. Quick Revision Table

Blood Group	Antigen	Antibody	Donate To	Receive From
A	A	Anti-B	A, AB	A, O
B	B	Anti-A	B, AB	B, O
AB	A & B	None	AB	All
O	None	Anti-A & Anti-B	All	O

37. Mnemonic Tricks

ABO Antibodies

A → Anti-B
B → Anti-A
AB → No antibodies
O → Both antibodies

Universal Donor “O gives to all.”

Universal Recipient “AB accepts all.”

38. Summary

Human blood groups are determined by genetically inherited antigens present on RBCs. The ABO and Rh systems are medically most important. Proper blood matching is essential to prevent dangerous transfusion reactions and hemolytic disease of newborns. Understanding blood groups is fundamental in physiology, genetics, immunology, and clinical medicine.

39. NCERT-Based One-Line Revision

Blood groups are determined by antigens on RBCs.
ABO system depends on A and B antigens.
Rh system depends on D antigen.
IA and IB are codominant alleles.
O group lacks antigens.
AB group lacks antibodies.
Rh incompatibility may cause erythroblastosis fetalis.
Anti-D injection prevents Rh sensitization.

Thursday, May 14, 2026

Human Blood Groups