ABO Blood Group System

Introduction to ABO Blood Group System

• ABO blood group system is crucial for blood transfusion and transplantation.
• Unique feature: individuals have antibodies against absent RBC antigens without prior exposure, leading to immediate lysis of incompatible donor RBCs.
• Incompatible transfusions can cause severe, potentially fatal transfusion reactions.

ABO Grouping Methods

• Forward Grouping (Front Type): Utilizes known antisera (anti-A, anti-B) to identify antigens on RBCs.
• Reverse Grouping (Back Type): Detects ABO antibodies in serum using known reagent RBCs (A1 and B cells).
• There is a reciprocal relationship between forward and reverse typing, serving as validation for results.
• Serum grouping is specific to the ABO blood group system.

Development of ABO Antibodies

• At Birth: ABO antibody production begins but is undetectable until 3-6 months due to low titers.
• Antibody levels peak between 5-10 years and decline in the elderly, possibly causing undetectable levels in reverse typing.
• Predominantly IgM antibodies that activate complement, react at room temperature.
• RBCs of newborns have 25-50% antigenic sites compared to adults, with full antigen expression by age 2-4.

Secretory Status

• Soluble ABH antigens found in body secretions; 80% of random U.S population are "secretors."
• Secretor status depends on inheritance of ABO genes and the secretor gene (Se or FUT2) on chromosome 19.
• FUT2 gene modifies the precursor substance to express A and B substances in secretions like saliva.

Molecular Genetics of ABO

• ABO gene located on chromosome 9 consists of 7 exons; exons 6 and 7 account for 77% of the gene.
• Mutations lead to A, B, or O specificities due to changes in enzyme functionality.
• A2 subgroup arises from specific mutations in the A2 gene affecting glycosyltransferase activity.

A Subgroups

• Majority of group A individuals are A1 or A1B; 20% are A2 or A2B.
• A2 has fewer antigen sites (240,000 to 290,000) resulting in less efficient conversion of H antigen to A antigen.

Characteristics of A2 Antigen

• Four types of H antigens characterized as unbranched or branched chains.
• A1 RBCs react with both anti-A and anti-A1, while A2 RBCs only react with anti-A and show more reactivity with anti-H lectin.
• A2 group exhibits occasional formation of anti-A1 antibodies.

Weak A and Weak B Subgroups

• Weak A subgroups are rare and identifiable through serological discrepancies.
• Characteristics: fewer A antigen sites, varying agglutination with anti-A, and variability in H antigen detectability.
• Weak B subgroups observed via reaction strength with anti-B and anti-A, B techniques.

Bombay Phenotype (Oh)

• Caused by inheritance of hh genotype, resulting in absence of ABH antigens due to lack of H antigen.
• Inherited as an autosomal recessive trait linked to mutations in FUT1 gene.
• Bombay phenotype RBCs do not react with anti-A, anti-B, or anti-H antisera, mimicking O blood type.

Antibodies in Bombay Serum

• Bombay serum contains anti-A, anti-B, anti-A,B, and anti-H antibodies capable of strong reactions at 37°C.
• Only compatible blood for transfusion comes from other Bombay phenotype individuals.

Para-Bombay Phenotypes

• Exhibit reduced or absent H antigens; genetic basis includes mutations in FUT1 and/or FUT2 genes.
• Individuals show very low levels of A or B antigens, undetectable by routine methods but identifiable using adsorption and elution techniques.

This quiz explores the ABO blood group system, which is fundamental in the context of blood transfusions and organ transplantation. Understanding this system is crucial for medical professionals and students alike. Test your knowledge on the different blood types and their significance!