| What is "Tissue typing"? |
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| Stem cells from bone marrow,
peripheral blood, or cord blood can be used for autologous or allogeneic
transplantation: |
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| • |
Autologous
transplantation uses the patient's own stem cells. |
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Allogeneic
transplantation uses stem cells from a donor. |
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| Donation can be given by
a related match (sibling), unrelated match (a volunteer donor outside
the family) or unrelated cord blood unit from the cord blood bank. |
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| For the transplant to be
successful, stem cells must match the patient's own cells as closely
as possible. In an autologous transplant, matching is perfect. However,
in allogeneic transplantation, the extent to which the donor's stem
cells match those of the patient must be investigated. The testing
process is based on tissue typing of 6 proteins, called Human Leukocyte
Antigens (HLA), that appear on the surface of white blood cells and
other tissues in the body. |
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| There are 2 classes of HLA: |
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| • |
Class I antigens (HLA-A, HLA-B,
and HLA-C) |
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Class II antigens (HLA-DR [and
HLA-DQ & HLA-DP]) |
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| Since each antigen has 2
alleles (forms), there are a total of 12 antigens. For stem cell transplants,
6 of these antigens - two HLA-A, two HLA-B and two HLA-DR antigens-
are considered most important for matching purposes. A 6/6 match is
said to occur when the HLA of the donor and recipient match with respect
to all 6 of these transplant-critical antigens. |
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| These two classes of antigens
vary in their structure and distribution among individuals and on
different body tissues. The donor's blood sample is typed for HLA
markers and the results are compared to the patient's HLA types for
matching. |
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| Although a perfect match
would be best, studies have shown that cord blood transplants are
successful even when only four of the six loci critical for a successful
stem cell transplant match. Cord blood is derived from a newborn whose
immune system is less mature than an adult's. As a result, a lesser
degree of matching is sufficient for cord blood transplant than from
adult blood or bone marrow thereby sometimes leaving cord blood as
the only available source for patients who otherwise lack a donor. |
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| Tissue typing is a very
costly procedure which can only be performed in specialized laboratories
using advanced serological and genetic techniques. The results are
submitted to national and international registries and are made available
for patients throughout the world. |
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