CLASS 1 TISSUE TYPING -
Noun: This is a procedure of examining the genetic markers on the surface of white blood cells called the HLA - antigens (Human leukocyte antigens). These antigens help the body identify invading organisms, and trigger an immune system attack on any substances that do not belong in that particular person's body, such as viruses and bacteria.

If the patient's and donor's HLA-antigens do not match, the patient's body will perceive the donor's bone marrow as foreign material to be destroyed. The new white blood cells being produced by the new bone marrow will also see the patient's body as something to be destroyed. These conditions are called graft rejection and graft-versus-host disease and both result in a failed bone marrow transplant .

There are two levels of determining the HLA genetic markers. This is Class 1 Tissue typing which gives 3 categories which identify where the tested genes are on the chromosomes, the 'A' locus , 'B' locus, and "C" or 'Cw' locus. This would be done first to determine prospective donors. There are 22 different 'A' locus antigens, 42 different 'B' antigens and 9 different Cw antigens.

If these match, then the Class 2 Tissue typing would be done to further ensure as close a match as possible. This has the HLA category of 'DR' and within that category there is also 'DQ' and 'DP'. There are 18 different 'DR' antigens. The number of combinations is vast but unrelated people with identical combinations occur at a rate of 1 in 50,000. Brothers and sisters at a rate of 1 in 4.

Lymphocytes will attack all cells that do not have the correctly matching antigens which is why it is vital to get as close a match as possible.

Although both the Class 1 and the Class 2 Tissue typing may be a perfect match, it is preferable to have the matching bone marrow from a brother or sister. The reason for this is that there are many other aspects in the genes which are not tested for or perhaps not even known about which if from an unrelated source may be completely unmatched. From a brother or sister, if the match is good on the Class 1 and 2, the chances of the untested and unknown genetic material matching is better resulting in a much less chance of GVHD being serious. We know this as the GVHD occurs with those that have perfectly matched bone marrow from a stranger much more frequently than it does with perfectly matched bone marrow from a family member.

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