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HIV: see Human Immunodeficiency Virus

HLA: The acronym for human leukocyte antigens. These proteins are on the surface of most tissue cells and give an individual his or her unique tissue type. HLA factors are inherited from mother and father and the greatest chance of having the same HLA type is between siblings. On average one in four siblings is expected to share the same HLA type. The testing for HLA antigens is referred to as “tissue typing.” There are six major groups of HLA antigens A, B, C, D, Dr, and Dq. These proteins on the surface of cells act as antigens when donated (transplanted) to another individual, the recipient. If the antigens on the donor cells are identical (e.g. identical twins) or very similar (e.g. HLA matched-sibling) the transplant (donated stem cells) are more likely to survive (engraft) in the recipient. In addition, the recipient's body cells are less likely to be attacked by the donated immune cells (graft versus host disease).

Hairy Cell Leukemia: An uncommon type of Chronic Lymphocytic Leukemia. The leukemic cells are B-lymphocytes that have characteristic "hair- like" projections when examined under the microscope. The spleen is frequently enlarged and the blood cell numbers are usually markedly decreased. The disease is very responsive to treatment with cladribine or pentostatin.

Haplotype: This is the tissue type contributed by either the mother or father to their offspring. It is implied that it represents the genes on one set of parental chromosomes. When a transplant procedure is between a donor and recipient who are haplotype identical, it means that the tissue type or HLA type of each is identical in respect to mother or father but not identical to the other. In some situations, if the discrepancy is not too great the transplant may still be possible if the underlying disease makes the risk of partial compatibility warranted. Conditioning of the recipient and lymphocyte depletion of the donor stem cell suspension are steps taken to mitigate the risk of immune cell activation by the tissue type differences.

Hemapheresis: The process of removing a donor's blood to extract a specific component and returning the unneeded parts to the donor. The process uses continuous circulation of blood from a donor through an apparatus and back to the donor. This process makes it possible to remove desired elements from large volumes of blood. Platelets, red blood cells, white blood cells, or plasma can be removed, separately. For example, this technique permits the harvest of enough platelets for a platelet transfusion from one donor (rather than six to eight separate donors). In so doing, the recipient of the platelets is exposed to the blood of fewer donors or can be given HLA-matched platelets from a single related donor. This technique is also used to remove circulating blood stem cells that can be frozen, stored, and, later, used instead of marrow stem cells for transplantation. The system of hemapheresis is closed and sterile.

Hematocrit: The proportion of the blood occupied by the red blood cells. Normal values are 40-54% in males, 35-47% in females. If the hematocrit is below normal, one has anemia. If the hematocrit is above normal, one has erythrocytosis.

Hematologist: A physician who specializes in the treatment of blood cell diseases. This person is either an internist who treats adults or a pediatrician who treats children. Hematopathologists are pathologists who specialize in the diagnosis of blood cell diseases and who perform the specialized laboratory tests, often required to make a conclusive diagnosis.

Hematology: The study of blood diseases, including leukemia, lymphoma, and myeloma.

Hematopoiesis: This term describes the process of blood cell development in the marrow. The most undeveloped cells in the marrow are stem cells. They start the process of blood cell development. The stem cells begin to develop into young or immature blood cells like red cells or white cells of various types. This process is called “differentiation.” The young or immature blood cells then further develop into fully functional blood cells. This process is called “maturation.” The cells then leave the marrow and enter the blood and circulate throughout the body. Hematopoiesis is a continuous process that is active normally throughout life. The reason for this continuous activity is that most blood cells live for short periods and must be continuously replaced. After release from the marrow, red cells are removed in four months, platelets in 10 days and most neutrophils in one to three days. About five hundred billion blood cells are made each day. This requirement for very rapid replacement explains the severe deficiency in blood cell counts when the marrow is injured by replacement with leukemia, lymphoma or myeloma cells.

Hemoglobin: The iron containing pigment in red blood cells that carries oxygen to the tissue cells. A reduction in red blood cells decreases the blood hemoglobin. A decreased blood hemoglobin concentration is called anemia. The decrease in hemoglobin concentration decreases the oxygen carrying capacity of blood. If severe, this decreased capacity may limit a persons capacity to exert themselves. Normal values of blood hemoglobin are 12 to 18 grams per 100 ml of blood. Healthy women have on average about 10 percent less hemoglobin in their blood than men do.

Hemorrhage: Bleeding either to the outside through the skin or internally.

Hepatitis: Inflammation of the liver. One of several viruses that infects liver cells often causes the disorder.

Hepatomegaly: The term used to indicate an enlargement of the liver.

High-Grade Lymphoma: The term is used to include several subtypes of lymphoma that progress relatively rapidly if untreated. These subtypes include AIDS-associated lymphoma, anaplastic large cell lymphoma, Burkitt's lymphoma, diffuse large cell lymphoma, and lymphoblastic lymphoma. Although these represent more rapidly progressive lymphomas, some are also among those that respond very well to treatment with combinations of drugs.

Histiocytes: Histiocytes is a synonym for macrophages. These cells are found in numerous tissues including the skin, liver, lung, lymphatic glands, spleen, bones and the brain. They are derived from blood monocytes after the latter leave the blood and enter tissues.

Histiocytosis: A disease in which histiocytes are the principal cells involved. Langerhans cell histiocytosis is a neoplastic proliferation of histiocytes that involves the skin, bone, brain and other organs.

host T: The recipient of a stem cell transplant.

Human Immunodeficiency Virus (HIV): The agent that leads to the development of the acquired immunodeficiency syndrome (AIDS). Individuals infected with the virus have an increased risk of developing lymphoma. The lymphomas are of the B-cell type and may involve the brain or be widespread at the time of occurrence. Patients carrying the virus become immunologically impaired and are susceptible to unusual infections that are caused by microbes that usually do not infect healthy persons. (See Opportunistic Infection)

Human T-lymphocytotropic Virus (HTLV): A family of viruses that invade T-lymphocytes. HTLV-1 produces a disease called adult T-cell leukemia or lymphoma primarily in Southern Japan and the Caribbean . HTLV-3 has been renamed the human immunodeficiency virus and is the cause of acquired immunodeficiency disease.

Humoral Immunity: The ability of the immune system to produces antibodies against antigens. Humoral immunity is produced by B-lymphocytes and their derivative, plasma cells. Plasma cells are the principal antibodies forming cells. The cells are scattered throughout the lymph nodes, other lymphatic tissue, and the bone marrow. When the B-lymphocyte make contact with an antigen they become activated, transform into plasma cells and make antibodies that react only with that antigen. (See Cellular Immunity)

Hypercalcemia: An abnormally high concentration of blood calcium. In myeloma, the breakdown of bone, which is rich in calcium, is the main cause of high blood and urine calcium. The high calcium can contribute to weakness, loss of appetite, nausea, confusion, lethargy, and other symptoms.

Hypereosinophilic Syndrome: A condition in which the number of eosinophils in the blood is markedly and persistently elevated with no apparent cause, such as an allergic disease or a parasitic infection. These patients have either an inflammatory disease, the hypereosinophilic syndrome (HES) or chronic eosinophilic leukemia.

Hyperleukocytosis: The term applies to patients with leukemia who at the time of diagnosis have extreme increases in their white blood cell count. This circumstance occurs most frequently in patients with chronic myelogenous leukemia. If severe enough, blood flow may be impaired by the very high concentration of immature leukocytes. Urgent treatment by hemapheresis and chemotherapy is usually administered if symptoms are severe.