For a blood donation, the blood group of the donor and recipient must be the same. 

With blood stem cell donation it's similar, but there is a much more complex system behind it: Here the match of the tissue markers (HLA-markers) is the key to a successful blood stem cell transplantation.

If the differences are too big, there is a risk that the donated cells will reject the body of the recipient because they see its tissue markers as "foreign". Conversely, there is also an increased risk that the transplanted cells will reject the recipient's body. 

In contrast to the four blood groups (A, B, AB and 0), there are billions of different combinations in the "HLA"-system 

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The HLA-system is a very complex system which determines the compatibility of genetic materials that are found in the cells of many tissues, especially in white blood cells (leukocytes).

The HLA-antigens are important tissue markers that differentiate between «foreign» and «self» and are inherited half from the mother and half from the father.  With their composition, billions of different combinations are today known.

The HLA-antigens are divided into classes: Class I consists of the antigens HLA-A, -B and -C; ones that are on the surface of our bodily cells, for example, our organs.  Class II consists of HLA-DR, -DQ and -DP; they are located on the surface of our immune cells, for example, the white blood cells.

In blood stem cell transplantation, the most important HLA-antigens and their sub-groups of both donor and recipient have to match. If this is not the case, the recipient’s body will reject the cells.