**1. Introduction**

Haematopoietic stem cell transplantation (HSCT) is currently used to treat many bone marrow and blood disorders as well as malignancies and has become either an established life‐saving treatment or a life‐sustaining option for many patients. However, even nowadays, HSCT has

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

a high treatment‐related mortality, as only half of the patients that receive a transplant will survive the procedure. This high mortality is due to major complications such as relapse, graft failure, delayed immune reconstitution, opportunistic infections and graft‐versus‐host disease (GvHD).

After allogeneic HSCT, donor natural killer (NK) cells and T cells can attack the allogeneic tumour in a phenomenon described as graft versus leukaemia (GvL), which is the beneficial aspect of tissue disparity. However, allografts contain lymphocytes able to recognise and respond against host antigens and cause potentially life‐threatening GvHD affecting internal organs and systems. In addition, GvHD can affect the skin and eyes and can be very painful and distressing for the patient, thus impacting on the patient quality of life especially in the case of chronic GvHD. Moreover, GvHD increases the risk of a variety of bacterial, viral and fungal infections [1].

Hal Broxmeyer proposed the use of umbilical cord blood (UCB) as a source of haematopoietic stem cells (HSCs) for transplantation in 1982. The first umbilical cord blood transplantation (CBT) was performed for the first time in 1988 to treat a patient with Fanconi's anaemia [2]. Since then, UCB has been increasingly used as a source of HSC extending the availability of allogeneic HSCT to patients who would not have a matched donor. UCB has some advantages over the use of other grafts such as faster availability, less stringent human leucocyte antigen (HLA) matching, decreased incidence and severity of GvHD. However, patients who receive a UCB transplant are at higher risk of graft failure, infection and delayed engraftment and immune reconstitution. In addition, the use of UCB is restricted by the number of cells it contains, in particular of HSC, and it does not offer the option of a donor lymphocyte infusion if needed.

However, UCB does not only contain HSC enabling transplant but also contains immune cells such as regulatory T (Treg) cells and numerous proteins involved in inducing tolerance during pregnancy to prevent foetal rejection. Therefore, UCB is a unique resource that could also be used in order to develop immunotherapeutic approaches to counteract complications post‐ HSCT including post‐UCBT. This chapter focuses on the latest studies and immunotherapeutic approaches relating to the use of UCB to treat GvHD post‐HSCT.
