Tissue Typing

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Tissue Typing
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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).

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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).

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• 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.

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• The closer the HLA antigens on the transplanted
  organ match the recipient, the more likely that
  the recipients body will not reject the
  transplant.
• For this reason, tissue typing of the kidney
  donor and recipient is necessary before a kidney
  transplantation.

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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)

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• 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

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• 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.

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• 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.

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• When performing an HLA typing test for a kidney
  transplant, the following HLA antigens are looked
  at
• HLA-A
• HLA-B
• HLA-DR

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• 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).

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• 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).

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• By analyzing which six of these HLA-antigens 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.

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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
• Patients DNA is isolated.
• PCR is used to amplify specific HLA genes.
• Genes are sequenced to determine which alleles
  are present.

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• 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 recipients serum is
  mixed with cells from 60 different individuals
  (each test is done separately).

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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 persons PRA, the less likely he/she
  is to reject a transplant.

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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
  donors white cells is mixed with a small amount
  of the recipients serum.
• By exposing the donors HLA to the recipients
  serum, scientists can determine if the recipient
  has antibodies to any of the donors HLA.

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• Positive Crossmatch A reaction between the
  donors and recipients samples occurs.
• Indicates that the recipients body will likely
  reject the implanted kidney.
• Indicates the transplant cannot be performed.
• Negative Crossmatch No reaction between the
  donors and recipients samples occurs.
• Indicates that the recipients body will most
  likely not reject the implanted kidney.
• Indicates the transplant can be performed.