For a bone marrow transplant to succeed the ‘tissue type’ of a donor and patient must match very closely. The best source of matching tissue is from the donor’s family : the chance of a brother or sister having the same tissue-type is one in four
30% of patients needing a transplant identify a donor with matching tissue-type within their own family and receive bone marrow from a relative, usually a sibling.
70% need to find an unrelated donor.
Tissue type is inherited and classified by a group of protein markers known as Human Leucocyte Antigens (HLA).
Certain white cells (known as T-cells) are responsible for distinguishing harmful ‘foreign’ cells from the body’s own cells - ‘host’ cells; and organising the body’s immune system to render the invading organism harmless.
In a successful transplant, the patient’s and donor’s cells will be so closely matched that they both identify each other as ‘host’ cells.
If the patient’s T-cells identify the grafted donor cells as ‘foreign’, the donated cells will be destroyed, the bone marrow will not engraft and the transplant will be unsuccessful.
Conversely, the donor’s T-cells may identify the patient cells as ‘foreign’ and start an immune system attack on the patient - a condition called graft-versus-host disease which, in severe cases, can be life-threatening.
The tolerance of each individual to mismatched cells varies, making the task of deciding whether to proceed without a perfect match critical, and still imprecise.
