In case of finding a compatible haematopoietic stem cell source, the patient’s haematopoietic stem cells will be destroyed by chemotherapy and/or by radiation. The transplantion itself is very simple. The bone marrow or the umbilical cord blood (all in a fluid state) is infused into the patient’s vein. The haematopoietic stem cells circulate in the blood stream, when they get to the bone marrow they settle down in it, they start to multiply and produce blood cells.

          However, the most difficult period starts after the transplant itself. By removing the haematopoietic cells, the patient’s immune system will be removed as well. The patient is therefore, for several weeks, endangered by any trivial infection, which can cause death. Thus, the patient must be in a sterile environment all the time. The problem is, that the cytomegalovirus infection (80% of adults carry this infection, but the immune system doesn’t allow it to manifest itself) is very often established in the donor’s bone marrow. This virus can cause pneumonia in the transplanted patient, which is a frequent cause of the patient’s death. The advantage of umbilical cord blood is that the virus is not present in there, because it’s too big to get through the placenta, i.e. from the mother’s to the child’s blood.

          In the case of the donor's (relative or non-relative) haematopoietic stem cells, a major risk is the progress of acute post-transplant reaction (graft versus host reaction). The transplanted foreign haematopoietic stem cells create a foreign immune system. The new immune system can actually attack any patient’s organ at anytime. This graft versus host reaction must be suppressed by immuno-suppressive medication. This however, generally decreases the defence capacity of the new immune system and the patient is threatened by infections in a long term.

          At first sight, haematopoietic stem cells transplantation looks very simple, nevertheless, haematopoietic stem cells transplantation from a donor is the most complicated therapy known. Also, due to this, it is simultaneously the most expensive therapy known. It is however compensated by the result – if the transplantation is successful, the patient, otherwise condemned to death, is cured.

          The transplantation of own haematopoietic stem cells (autologous transplantation, autotransplantation) is much easier, as there is no risk of graft versus host reaction.