The
ABO
gene
has
three
alleles
-
A,
B,
and
0
-
with
A
and
B
dominant
over
O.
Since
man
is
diploid,
the
two
alleles
per
individual
provide
the
six
genotypes
and
four
phenotypes
as
follows:
Four
major
blood
types
are
thus
known:
A,B,AB,
and
O.
Each
type
is
distinguished
by
certain
antigens
on
the
surface
of.
the
erythrocytes
and
certain
antibodies
in
the
plasma.
Before
transfusion,
it
is
important
that
the
blood
types
of
participants
are
determined
so
that
incompatible
types
are
not
mixed.
For
instance,
if
a
Type
A
blood
donates
to
a
recipient
whose
Type
is
0
the
A
antigens
on
the
donor's
erythrocytes
will
react
with
a
antibodies
in
the
recipeint
plasma
and
the
cytotoxic
effect
will
be
expressed
as
agglutination
of
donor
erythrocytes
in
the
recipients
circulatory
system.
If
the
conditions
are
reversed,
the
donor's
A
antibodies
will
react
to
a
lesser
degree
with
the
recipient's
A
antigens,
because
dilution
in
the
recipeint's
plasma
takes
place.
The
older
notions
that
Type
0
blood
is
the
universal
donor
and
that
type
AB-
is
the
universal
recipient
are
mostly
untenable.
It
is
always
desirable
to
cross-match
the
donor's
erythrocytes
with
the
recipient's
plasma
as
well
as
the
reverse,
to
ensure
compatibility.
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