Biology today

The abO locus controls the type of glycolipids found on the surface of erythrocytes, apparently by specifying the type of glycosyl-
transferases (enzymes catalysing the synthesis of polysaccharides). The specific types of glycolipids on the red blood cell surface
provide the antigenic determinants that react with specific antibodies present in the blood serum.

When type a blood and type b blood are mixed, the anti-a antibodies in the type b blood serum react with the antigens on the type
a blood cells and vice versa, which causes agglutination or clumping of cells.

Cross-matching of blood types to determine compatibility is thus essential in blood transfusions. In this process, blood donors
and recipients are tested for the presence of antigens and antibodies that are incompatible. Individuals with blood type ab have
both a and b antigens on their erythrocytes, but no anti-a and anti-b antibodies in their blood serum. Hence, referred to as
universal recipients. Type O individuals lack both antigens, but carry both anti-a and anti-b antibodies in their blood serum.
Type O individuals are referred to as universal donors. Type O blood can be used in transfusion for individuals of any blood type
if the blood is introduced slowly enough to permit sufficient dilution of the anti-a and anti-b antibodies present in the serum of the
donor.

Phenotype

(Blood

group)

Genotypes Antibodies

present in

blood serum

Results from adding red blood cells from groups below to serum

from groups at left

A B AB O

a IAIA or IAi anti-b

b IBIB or IBi anti-a

ab IAIB –

O ii anti-a

anti-b

No agglutination

agglutination/clumping

Bombay Phenotype

In 1952, a very unusual situation provided information concerning the genetic basis of the H substance (polysaccharide precursor

for a and b antigens). In need of a transfusion, a woman was found to lack both a and b antigens and was thus typed as O.

However, one of her parents was type ab and she was the obvious donor of an Ib allele to two of her offspring. Thus, she was

genetically type b, but functionally type O.

The bombay woman was subsequently shown to be homozygous for a rare recessive mutation, h, which prevented her from

synthesising the complete H substance. In this mutation, the terminal portion of the carbohydrate chain protruding from the red

cell membrane was shown to lack fucose.

In the absence of fucose, the enzymes specified by the IA and IB alleles apparently are unable to recognise the incomplete

H substance as a proper substrate. Thus, neither the terminal galactose nor N-acetylgalactosamine can be added, even though

the appropriate enzymes capable of doing so are present and functional. as a result, the abO system genotype cannot be

expressed in individuals of genotype hh, and they are functionally type O. To distinguish them from the rest of the population,

they are said to demonstrate the bombay phenotype. The frequency of the h allele is exceedingly low. Hence, the vast majority of

the human population is the HH or Hh genotype (almost all HH) and can synthesise the H substance.