Page 367 - Concise Pathology for Exam Preparation ( PDFDrive )

352 SECTION II Diseases of Organ Systems

Q. What is the clinical significance of the ABO blood group system?

Ans. The ABO system (Table 12.23) is the product of one gene locus situated on chromo-
some 9, which determines the expression of ABO blood groups on RBCs, endothelial cells
and some epithelial cells.
• The basic precursor substance in ABO antigens has a short chain of sugars.
• There are two types of chains; the Type I and Type II chains, which differ from each
other in the way the terminal galactose joins the N-acetyl glucosamine residue.
• The basic precursor substance is converted to H substance by L-fucosyltransferase (H
gene codes for this transferase that attaches fucose to the terminal end of the precursor
substance to produce H antigen).
• The A gene codes for a transferase that attaches N-acetylgalactosamine to the precur-
sor substance, thereby producing A antigen (blood group A).
• The B gene codes for a transferase that attaches galactose to the precursor substance to
produce B antigen (blood group B).
• The O gene is inactive; hence, neither A, nor B antigens are present on the surface of
blood group O RBCs.
• Group AB individuals have H antigen that carries both A or B active sugars.
• An individual receives one blood group antigen from the mother and one from the
father.
• Antibodies belonging to ABO system are naturally occurring, IgM type, complete
antibodies (capable of agglutinating RBCs in saline suspension).
• The A group contains about twenty subgroups, of which A1 and A2 are the most
common. A1 makes up 80% of all A type blood while A2 makes up for the rest.

TABLE 12.23. ABO blood group
Blood group Antigen on cell surface Antibody in serum
A A Anti-A
B B Anti-B
AB A and B Neither
O Neither anti-A or anti-B Anti-A, B

Bombay Phenotype (hh)
Some individuals do not inherit the H gene and are not able to express substance H on
their RBCs and thus, do not produce A or B antigens. Instead, they produce antibodies to
substance H and both A and B antigens. They can receive blood only from other hh donors
but can donate like group O individuals.

Routine ABO Grouping
• Includes both cell and serum testing
• ABO testing should be done at room temperature or lower
• Controls should always be run during ABO grouping

Two Types of Grouping
• Forward type identifies the blood group antigen on the surface of RBCs by using anti-A
and anti-B test serum.
• In reverse grouping, group A and B red cells are allowed to react with patient serum to
identify the isohaemagglutinins that correspond with the blood group.
• Before transfusion, the ABO system must be appropriately matched between recipient
and donor.
• For example, a blood group A person, who has anti-B IgM antibodies, can receive only
A or O blood.

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