ABO System
SUBMITTED TO:
Ma’am Madiha Tariq
SUBMITTED BY:
NOMAN HAFEEZ KHOSA
SUBJECT:
Human Physiology
DISCIPLINE:
BS-MICROBIOLOGY
DATED: (JUNE10, 2014.)
ABO ANTIGENS
In
the 1901, Karl Landsteiner discovered the ABO blood group antigens. By systematically
mixing the RBC from a number of individuals with the sera from others, he found
that the RBCs from some individuals were agglutinated by the sera from others.
A pattern of four major groups emerged- A, B, AB, or O. Individuals have either
A or B antigen on their cells, a combination of A&B, or neither (group O). Subtle
differences distinguish the A and B blood group antigens. An individual lacking
one or both of these antigens will have serum Abs tothe missing Ag(s). A type O
individual is missing both A and B Ags on the cell and therefore has anti-A and
anti-B Abs in the serum.
ABO and H Blood Groups
Development of the A and B Antigens
1.
The A and B antigens are not fully developed in newborn infants, even though
some antigens can be detected on the red cells of the embryo as early as five
weeks after conception.
2.
Weaker agglutination reactions are observed with fetal and newborn infants'
RBCs compared to the mature RBCs of adults due to the number and strength of A
and B antigen sites being less.
Biochemical Activities Related to the
Development of A, B and H Antigens
1.
Inheritance of A and B genes usually results in the expression of A and B gene
products (antigens) on RBCs, but H, A and B antigens are not the direct
products of the H, A and B genes.
2.
Each gene codes for the production of a specific transferase enzyme
which catalyzes the transfer of a monosaccharide molecule from a donor
substrate to a predetermined precursor substance.
3.
The H gene codes for the production of fucosyl transferase that catalyzes the
addition of L-fucose, the immunodominant structure of H antigen, to two
slightly different structures, known as the type 1 and type 2 precursor chains.
The H gene and its allele h are inherited independently of the allelic A, B and
O genes.
4.
Once the H gene-specified transferase has acted and the L-fucose has been added
to the two chains, the A and B gene-specified products can act to add sugars to
the chains that now carry H.
5.
The A gene codes for production of a galactosaminyl transferase that effects
the addition of Nacetylgalactosamine to the preformed H-bearing chains.
6.
The B gene codes for production of a galactosyl transferase that effects the
addition of D-galactose to the same H-bearing structure.
7.
Thus, the immunodominant structure of the H antigen is L-fucose, of the A
antigen Nacetylgalactosamine and of the B antigen, D-galactose.
8.
In the absence of L-fucose, the immunodominant structure of H, the A and B
immunodominant sugars cannot be added. In other words, if an individual does
not inherit a functional H gene, the A and B immunodominant sugars cannot
be added to the structures that normally carry those determinants. This
is the basis of the explanation of the Bombay or Oh phenotype.
a. When
an individual inherits two h genes, h being a rare allele of H, the A and B immunodominant
structures are not added. The h gene is an amorph with no detectable product.
b. In
spite of the dependence of A and B antigen assembly on the presence of H at the
biochemical level, the Hh genes are not part of the ABO system. Independent
segregation of genes at the Hh and ABO loci has been demonstrated in several
families.
c. In
individuals who inherit two h genes, A and B gene function is not blocked. In
other words, the A and B gene-specified transferase enzymes are still produced
(dependent, of course, on the inheritance of an A or B or both genes) but
because of lack of H (L-fucose) on the type 1 or type 2 precursor chains,
cannot add acetylgalactosamine or galactose (respectively) to those chains.
Oh Phenotype (Bombay)
1.
This phenotype occurs when two hh genes are inherited at the Hh locus.
a. These
individuals possess normal A or B genes but are unable to express them because
they lack the gene necessary for production of H antigen, the required
precursor for A and B.
b. These
individuals will transmit the normal A or B gene to offspring.
2.
The term "Bombay" used for Oh phenotype because examples
of such RBCs were first discovered in Bombay, India.
3.
Symbol Oh denotes the phenotype because results obtained in ABO
grouping mimic those of group O persons.
a. The
RBCs are not agglutinated by anti-A, -B or -A,B.
b. The
serum agglutinates the reverse A and B cells.
4.
Generally not recognized until the serum is tested against group O cells in the
antibody screen and agglutinates all O cells tested.
a. Because
these individuals lack A, B and H antigens, they form potent anti-A, -B, -A,B
and anti-H, which is the most clinically significant.
b. These
individuals can only be transfused with Bombay blood which occurs
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c.
5. Confirmatory testing for Oh
a. Test
patient with anti-H lectin Ulex europaeus. Normal group O cells
are strongly
agglutinated
while Oh cells are not agglutinated.
b. The
patient's serum will agglutinate all blood types (A, B, AB and O).
c. The
patient's serum will fail to agglutinate Oh cells.
The Secretor genes
1.
The A, B and H antigens are not confined to red cells but may be present in
body fluids also.
2.
The secretion of A, B and H substances in saliva and other body
fluids is controlled by a pair of
alleles,
Se and se, called the secretor genes.
3.
Secretion of A, B and H soluble substances is accomplished even when only one
locus carries Se.
4.
There can be no Se when se is present on both chromosomes. The gene se is an
amorph.
5.
The secretor genes are not linked to the ABO locus, they are inherited
independently.
6.
Persons who have A, B and/or H substances in saliva are called secretors and
the following will be present in their saliva:
Blood Group
|
Substances in Saliva
|
A
|
A
and H
|
B
|
B and H
|
AB
|
A,B
and H
|
O
|
H
|
7.
Provided the person is a secretor (whether Se/Se or Se/se), saliva tests can be
helpful in defining a subgroup or in resolving the genetic makeup of an
individual who appears to have an unusual blood group.
8. About
80 percent of Caucasians are secretors.
O. ABO Discrepancies
1. A
discrepancy exists when the interpretation of the forward type does not
correlate with that of the reverse type.
2.
ABO interpretations must be delayed until the discrepancy is resolved. If
emergency transfusion is indicated give group O RBCs of the appropriate D type.
3.
Technical errors causing false negative reactions may be caused
by:
a. Failure
to add serum or antiserum to a test.
b. Failure
to identify hemolysis as a positive reaction.
c. Not
using the appropriate serum (or reagent) to cell ratio.
d. Improper
centrifugation.
e. Incubation
of tests at temperatures above 20-25 C.
f. Use
of inactive reagents.
g. Failure
to interpret or record test results correctly.
4.
Technical errors causing false positive reactions may be caused
by:
a. Over-centrifugation
b. Use
of contaminated reagent antibodies, RBCs or saline.
c. Use
of dirty glassware.
d. Incorrect
interpretation or recording of test results.
5.
Problems associated with testing red blood cells (forward type).
a. Samples
obtained from a recently transfused patient or a bone marrow transplant
patient.
b. Persons
who are an ABO subgroup or who have weakened antigens due to diseases
such as leukemia.
c. High
levels of abnormal proteins or Wharton's jelly (cordbloods) which may cause nonspecific
aggregation.
d. High
concentrations of A or B blood group substances in serum may on rare occasions
inhibit activities of blood group reagents to such an extent to give a false
negative with unwashed cells.
e. Sera
of some individuals contain antibodies to dyes used to color anti-A or anti-B
causing a false positive when unwashed cells are used.
f. Individuals
with potent cold autoagglutinins may coat their own RBCs so heavily that they spontaneously
agglutinate.
6.
Problems associated with the serum testing (reverse type).
a. Small
fibrin clots.
b. Rouleaux
c. Patients
with abnormally high levels of abnormal serum proteins or who have received
plasma expanders.
d. Antibodies
other than ant-A or anti-B.
e. Antibodies
to chemical constituents of the diluents used to preserve the reverse cells.
f. Negative
or weak reactions on specimens from infants 4-6 months of age.
g. Bone
marrow transplant from ABO non-identical donor.
h. Very
weak or negative reactions from patients with immunodeficiency due to disease,
therapy, depressed immunoglobulin levels, elderly patients, or patients who
have received large amounts of IV fluids.
i. Unexpected
reactions when patient receives sufficient volumes of blood components containing
plasma of an ABO group other than their own.
Resolving ABO
Discrepancies
1.
First course of action should always be to repeat testing on a better washed
RBC sample and using serum from the original specimen.
2.
If discrepancy persists:
a. If
patient appears to be group A test the RBCs with anti-A1 lectin and serum with
A2 cells. If the patient is negative with the lectin they are of the subgroup
A2, and if the serum is negative with the A2 cells, this shows they have
anti-A1 in their serum.
1) Most commonly encountered
discrepancy.
2) Test with additional A2 cells to confirm
specificity due to anti-A1.
b. Incubate
at RT for 30 minutes for detection of weakened antibodies or antigens.
Frequently needed for elderly patients. Can also incubate at 4 C but must
run an autocontrol. Cold antibodies are frequently encountered in tests
performed at 4 C.
c. Test
the serum against group O adult, group O cord and a patient autocontrol
(patient serum plus patient cells) to determine if cold reactive antibodies are
interfering with testing (also fairly frequently encountered).
d. Wash
the patient and reagent RBCs several times.
e. Obtain
a new specimen.
3.
Acquired B Phenotype
a. Patient's
RBCs appear to be group AB with a weak B antigen, yet the serum contains
anti-B.
b. This
phenomena appears to be associated with carcinoma of the colon or rectum,
infection with gram negative organisms and intestinal obstruction.
c. This
phenomena has also been associated recently with some monoclonal anti-B typing
sera.
4.
Mixed field agglutination, a term used to describe discrepancies associated
with samples which have two distinct cell populations. May be due to:
a. A or
B patients transfused with group O blood.
b. D
positive patents transfused with D negative blood or D negative patients
transfused with D Positive blood.
d. The
subgroup A and B classically exhibits a mixed field agglutination 3 3 when
tested with anti- A, this aids in its identification.
e. The
rarest form is due to chimerism due to an intrauterine exchange of
erythropoietic tissue by fraternal twins.
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