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Tissue Typing
Everyone has several antigens
located on the surface of his/her
leukocytes:
• One particular group of these
antigens is called the HLA (Human
Leukocyte Antigens).
The HLA
• Is responsible for stimulating the
immune response to recognize tissue
as self versus non-self.
• Is controlled by a set of genes located
next to each other on chromosome 6
called the Major Histocompatibility
Complex (MHC).
• The test that determines which HLA
antigens are present is called tissue
typing or HLA typing.
–Tissue typing identifies the similarity
of the antigens present in both the
donor and the recipient.
• The closer the HLA antigens on the
transplanted organ match the recipient,
the more likely that the recipient’s body will
not reject the transplant.
• For this reason, tissue typing of the kidney
donor and recipient is necessary before a
kidney transplantation.
There are two main classes of HLA
antigens:
• Class I (HLA-A, HLA-B, and HLA-Cw)
• Class II (HLA-DR, HLA-DQ, and HLA-DP)
• Every person inherits each of the
following antigens from each
parent:
– HLA-A antigen
– HLA-B antigen
– HLA-Cw antigen
– HLA-DR antigen
– HLA-DQ antigen and
– HLA-DP antigen
• The set of HLA antigens received
from a parent is called a
haplotype.
–There are a variety of alleles for
each of these HLA antigens.
–The large number of possible
variations and combinations of HLA
antigens make finding a match in a
family more likely than finding a
match in the general public.
• When performing an HLA typing test for a
kidney transplant, the following HLA
antigens are looked at:
– HLA-A
– HLA-B
– HLA-DR
• The MHC genes are the most polymorphic
known.
– There are hundreds of known alleles for each
HLA Antigen.
• Each allele is identified by a number (i.e. HLA-A1
or HLA-A2).
• Six HLA antigens are looked at for each person.
– Remember each person has two of each of
the antigens (one inherited from the mother
and one inherited from the father).
• By analyzing which six of these HLAantigens both the donor and recipient
have, scientists are able to determine the
closeness of tissue matching.
– A six-antigen match is the best compatibility
between a donor and recipient.
• This match occurs 25% of the time between
siblings who have the same mother and father.
HLA Typing Techniques
• Traditionally, HLA typing was done using
serological techniques:
– Blood from the patient was mixed with serum
containing known antibodies to determine which
antigens were present.
• HLA typing now is predominantly done using
molecular techniques:
– Patient’s DNA is isolated.
– PCR is used to amplify specific HLA genes.
– Genes are sequenced to determine which alleles are
present.
• Once the donor and recipient have been
tested for tissue compatibility, the next
step is an Antibody Screening (also called
a Panel Reactive Antibody or PRA).
– A small amount of the organ recipient’s serum
is mixed with cells from 60 different
individuals (each test is done separately).
Purpose of Antibody Screening
• Scientists can determine how many
different HLA antibodies a patient has in
his/her blood.
– If a patient reacts with 30/60 cells, he/she is
said to have 50 Percent Reactive Antibody
(also known as PRA).
– The lower a person’s PRA, the less likely
he/she is to reject a transplant.
Crossmatch Test
• After tissue typing and antibody screening
are complete and a potential donor has
been identified, the final test is called a
crossmatch test.
– Crossmatch Test: A small amount of the
potential donor’s white cells is mixed with a
small amount of the recipient’s serum.
• By exposing the donor’s HLA to the recipient’s
serum, scientists can determine if the recipient has
antibodies to any of the donor’s HLA.
• Positive Crossmatch: A reaction between the
donor’s and recipient’s samples occurs.
– Indicates that the recipient’s body will likely reject the
implanted kidney.
– Indicates the transplant cannot be performed.
• Negative Crossmatch: No reaction between the
donor’s and recipient’s samples occurs.
– Indicates that the recipient’s body will most likely not
reject the implanted kidney.
– Indicates the transplant can be performed.