Graft-versus-host disease (GvHD) is a complication that may arise after a bone marrow transplant is performed, in which the recently transplanted material attacks the body of the person who has received the transplant.

The immune system is what the body uses to defend itself from infection. In order to do this it recognises harmful cells as 'foreign' elements, and attacks them. When an unhealthy person receives stem cells from a donor (the graft), their task is to recreate the donor's immune system's functions in the patient's body (the host).

Differences between the donor's bone marrow and the receiver's tissues often lead to the donor's bone marrow T cells (a type of white blood cell) recognising the receiver's tissues as foreign. When this happens, the recently transplanted bone marrow attacks the patient's body.

Graft-versus-host disease can be acute or chronic. The greater the incompatibility, or incorrect pairing, there is between the donor and the patient, the greater the risk of GvHD. That is why it is of such importance for there to be more bone marrow donors so that, when a patient does not have a compatible donor in the family, another ideal donor may be found. After receiving a bone marrow transplant, the patient is generally administered drugs to inhibit the immune system in order to reduce the probability, or gravity, of graft-versus-host disease.

One of the main aims of scientific research into malignant blood diseases is to avoid graft-versus-host disease. The problem resides not only in having a better understanding of every disease's progress, but also in developing ways to control the most frequent complications that may arise because many patients die due to such complications, frequently of an infectious kind, rather than on account of the disease itself. This is one of the challenges for the Josep Carreras Leukaemia Research Institute.

A bone marrow transplant is one of the treatments of choice for various types of leukaemia, but leukaemia is not the only disease that might be cured through this procedure. In this document we want to provide information about the many illnesses that may be cured thanks to an allogenic bone marrow transplant (from a related or non-related donor) (1).

• Leukaemias

Acute leukaemias (acute lymphoblastic leukaemia - ALL, and acute myeloid leukaemia - AML) account for most of the cases for which the Bone Marrow Donor Registry initiates a search for a non-related donor. For example, in 2012, both types of leukaemia accounted for more than 45% of the searches made by REDMO.

Leukaemia consists in the uncontrolled proliferation of an anomalous population of blood cells. These anomalous cells infiltrate the bone marrow and prevent the production of other, normal, cells and they invade the blood and other organs.

It is usually the acute leukaemias that require a bone marrow transplant. Chronic leukaemia can, in many cases, be controlled with drugs.

Patients may need a transplant of hematopoietic progenitors if suffering from the following leukaemias: acute lymphoblastic leukaemia, acute myeloid leukaemia, post-myelodysplastic acute leukaemia, acute secondary leukaemia chronic lymphocytic leukaemia.

What proportion of leukaemia patients require a transplant?

Not all patients suffering from leukaemia require a hematopoietic progenitor allogenic transplant (from a donor). For example, only 10% of children suffering from acute lymphoblastic leukaemia (ALL) require this procedure. Furthermore, patients suffering from certain acute leukaemias such as, for example, acute promyelocytic leukaemia, do not need an allogenic transplant. This is also the case for most chronic leukaemias, such as chronic myeloid leukaemia. Bearing this in mind, together with the fact that, in principle, patients over the age of 65 can not receive a transplant of these characteristics, it can be said that some 20% of acute leukaemia patients require an allogenic transplant.

• Lymphomas

Lymphomas are a group of neoplastic diseases (cancers) that occur in the lymphatic system, which forms part of the body's immune system. There are two large groups of lymphomas: Hodgkin lymphoma (HL) and non-Hodgkin lymphoma (NHL). Patients with Hodgkin lymphoma do not usually receive a transplant from a donor because this disease can be held in check with chemotherapy and/or an autologous bone marrow transplant. Nevertheless, in cases where these treatments have not worked, a transplant from a family member or a non-related donor may be performed. In 2012, for example, 5% of all searches conducted by REDMO were for patients with Hodgkin lymphoma.

In the case of non- Hodgkin lymphoma, a certain type of lymphocyte reproduces uncontrollably causing, over time, an increase in the size of the organ in which they are produced. Given that lymphatic tissue is found all over the body, the lymphomas may appear in any part of the organism and spread to the liver and other organs and tissues. There are many different sub-types of non-Hodgkin lymphoma and some of them do require an allogenic transplant of hematopoietic progenitors. The subtypes of non-Hodgkin lymphoma that are most frequently treated with a transplant of hematopoietic progenitors are: B-, T- or NK-cell lymphoma; follicular lymphoma, extranodal marginal zone B-cell lymphoma ( MALT), lymphoplasmacytic lymphoma, diffuse large B-cell lymphoma, mantle cell lymphoma, Burkitt lymphoma, anaplastic lymphoma, peripheral T-cell lymphoma, mycosis fungoides or Sezary syndrome and hepatosplenic lymphoma.

What proportion of lymphoma patients require a transplant?

At the present time and in general, lymphomas, lymphoproliferative syndromes, are the most frequent cause for performing an autologous transplant of hematopoietic progenitors. In the case of diffuse large B-cell lymphoma (the most frequent histological type of lymphoma) the standard procedure, accepted by most transplantation organisations and scientific groups, includes performing a transplant on patients after a first relapse, something that occurs in almost 50% of patients with this lymphoma subtype. The procedure with the other histological subtypes depends on such a multitude of factors that the way in which a decision is made regarding a transplant must, in many cases, be made individually, on a case by case basis for each patient.

• Myelodysplastic syndromes

In the case of myelodysplastic syndromes (MDS) the blood stem cells produced by the bone marrow and responsible for producing all the cells in the blood, do not mature and consequently do not become healthy red blood cells, white blood cells or platelets.

In general, in the less severe cases of MDS, treatment is not required, and the disease can remain stable for years. The only treatment that can produce a cure is a bone marrow transplant, but the advanced age of many patients and the toxicity of the procedure limit its use to young patients with an MDS with a poor prognosis who have a compatible donor.

MDS patients who may require an allogenous bone marrow transplant are those with: refractory anemias with excess blasts, refractory cytopenia with multilineage dysplasia, chronic myelomonocytic leukaemia, and myelodysplastic syndromes that are difficult to classify.

What proportion of patients suffering from myelodysplastic syndromes require a transplant?

At the present time only a minority of patients with myelodysplastic syndromes require an allogenic transplant, which is only indicated for high-risk patients who are young and have a compatible donor, accounting for a quarter of cases.

• Bone marrow aplasia

Bone marrow aplasia consists in the disappearance from the bone marrow of the stem cells that are responsible for producing all blood cells. Patients with less severe aplasia may be practically asymptomatic and require no treatment. The treatment of choice in the case of severe and very severe cases in young patients is a bone marrow transplant.

• Congenital red blood cell disorders

Erythrocytes (also known as red blood cells and hematids), are the most numerous elements in the blood. Hemoglobin is one of their main components and its function is to transport oxygen to the various tissues in the body. Congenital anomalies of the erythrocytes that may lead to the patient to require a bone marrow transplant to seek a cure are: Fanconi anemia, Alfa-thalassemia, Beta- thalassemia major, drepanocytosis (sickle-cell disease) and sideroblastic anemias.

• Plasma cell alterations

Plasma cell alterations, or monoclonal gammopathies, are a group of diseases that includes various malignant blood diseases characterised by plasma cell disorders and immunoglobulin deficiency.

Malignant monoclonal gammopathies that may require a transplant of hematopoietic progenitors are: multiple myeloma, plasma cell leukaemia, Waldenström macroglobulinemia and amyloidosis.

What proportion of patients suffering from monoclonal gammopathies require a transplant?

Approximately a third of patients with multiple myeloma require an autologous transplant, 20% of patients with amyloidosis and 10% of patients with Waldenström macroglobulinemia.

• Congenital immunodeficiency

Immunodeficiency is a condition in which the immune system does not fulfil its function of protecting the body, thereby leaving the organism prone to infection. Primary (or congenital) immunodeficiencies manifest themselves, except in some circumstances, from infancy and are due to congenital defects that impede the correct functioning of the immune system.

There are different kinds of congenital immunodeficiency but they are mostly extremely rare. Perhaps the most frequent one in the group is severe combined immunodeficiency (SCID), a cure for which inevitably involves an allogenic bone marrow transplant.

• Other illnesses

It would be possible to mention many other diseases for which a transplant of hematopoietic progenitors is often the only treatment that may offer the possibility for a cure, but many of them are minority diseases, such as other neoplasms like primary myelofibrosis or idiopathic myelofibrosis; histiocytosis, lysosomal storage disease and other, non-neoplastic, diseases.

What proportion of patients with chronic myeloproliferative syndromes such as myelofibrosis require a transplant?

An allogenic transplant of hematopoietic progenitors is rarely performed for patients with chronic myeloproliferative syndromes.

(1) Obviously, the information provided can, in no way, be a substitute for medical consultation with a specialist doctor. It is the patient's hematologist who decides on the most appropriate treatment, bearing in mind the patient's age, physical condition, and the disease's progress.