Hematology 101

Blanche P. Alter, MD, MPH
Clinical Genetics Branch, Division of Cancer Epidemiology and Genetics,
National Cancer Institute, Bethesda, MD

A brief introduction to the terminology of hematology is provided here.

Blood cells are produced in the bone marrow, and enter the circulation after they are sufficiently mature. Blood contains plasma (the liquid), and cells. The cells consist of red cells, white cells, and platelets. Red cells are filled with hemoglobin, which carries oxygen from the lungs to the tissues. Red cells do not have a nucleus; it is lost before the maturing cells leaves the bone marrow. Reticulocytes are the newest red cells. They fail to increase in number if anemia is due to bone marrow failure, because of the inability of the marrow to respond. There are many different types of white blood cells. Lymphocytes form the immune system and consist of T-cells, which direct other cells and are responsible for cell-mediated immunity, and B-cells, which produce antibodies. The myeloid white cells are primarily phagocytes, which kill and digest bacteria. The most important are the neutrophils, also called granulocytes (because the cytoplasm contains granules which have enzymes which destroy bacteria), or polymorphonuclear or segmented cells (because the nucleus has several segments). Monocytes are another type of phagocyte. Eosinophils and basophils are important during allergic events. Platelets are particles derived from the cytoplasm of their bone marrow precursor, negakaryocytes. Red cells survive 120 days, platelets 7-10 days, lymphocytes months to years, and neutrophils only 6 - 12 hours.

Normal blood counts are age-dependent for Hb (hemoglobin) and MCV (mean cell volume of the red cells). In general, Hb <8g/dl, ANC<1500/mm(, or platelets<30,000/mm3 lead to consideration of therapeutic intervention, either stem cell transplantation, or medical treatment. ANC stands for absolute neutrophil count, and is obtained by multiplying the total white blood count by the % neutrophils. To evaluate the status of the bone marrow, we recommend an annual bone marrow aspirate to evaluate the types of blood cell precursors in the marrow, a biopsy to assess cellularity (the proportion of the marrow that contains cells rather than fat), and cytogenetics for evidence of clonal disease.

Aplastic anemia is defined as pancytopenia (decreased red cells, white cells, and platelets) with hypocellular bone marrow. Leukemia is defined as a malignant proliferation of immature cells, called "blasts." Myelodysplastic syndrome (MDS) is cytopenia with a hypercullular marrow, but with less than 20% blast cells. Significant MDS is associated with clinically important cytopenias, associated with dysplastic erythroid, myeloid, or megakaryocytes; the dysplastic features are characteristic of each cell lineage. Cytogenetic clones, while often seen in MDS and in acute leukemia, may not independently predict leukemia or an adverse outcome.

Reprinted from the Fall 2003 FA Science Letter