The hematopoietic and lymphoid systems are affected by a wide spectrum of diseases. One useful way to organize these disorders is based on whether they primarily affect red cells, white cells, or the coagulation system, which includes platelets and clotting factors. The most common red cell disorders are those that lead to anemia, a state of red cell deficiency. White cell disorders, by contrast, are most often associated with excessive proliferation resulting from malignant transformation. Derangements in blood coagulation may result in hemorrhagic diatheses (bleeding disorders). Finally, splenomegaly, a feature of numerous diseases, is discussed at the end of the chapter, as are tumors of the thymus.
Although these divisions are useful, in reality the production, function, and destruction of red cells, white cells, and components of the hemostatic system are closely linked, and derangements primarily affecting one cell type or component of the system often lead to alterations in others. For example, in certain conditions B cells make autoantibodies against components of the red cell membrane. The opsonized red cells are recognized and destroyed by phagocytes in the spleen, which becomes enlarged. The increased red cell destruction causes anemia, which in turn drives a compensatory hyperplasia of red cell progenitors in the bone marrow.
Other levels of interplay and complexity stem from the anatomically dispersed nature of the hematolymphoid system and the capacity of both normal and malignant white cells to “traffic” between various compartments. Hence, a patient who is diagnosed with lymphoma by lymph node biopsy also may be found to have neoplastic lymphoid cells in the bone marrow and blood. The malignant clone of lymphoid cells in the marrow may suppress hematopoiesis, giving rise to low blood cell counts (cytopenias), and the dissemination of tumor cells to the liver and spleen may lead to organomegaly. Thus, in both benign and malignant hematolymphoid disorders, a single underlying abnormality can result in diverse systemic manifestations. Keeping these complexities in mind, we will use the time-honored classification of hematolymphoid disorders based on predominant involvement of red cells, white cells, and the hemostatic system.
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Disorders of red cells can result in anemia or, less commonly, polycythemia (an increase in red cells also known as erythrocytosis). Anemia is defined as a reduction in the oxygen-transporting capacity of blood, resulting from a decrease in the red cell mass to subnormal levels.
Anemia can stem from bleeding, increased red cell destruction, or decreased red cell production. These mech anisms serve as one basis for classifying anemia (Table 12.1 ). In some entities overlap occurs, for example in thalassemia, wherebothreducedredcellproductionandearlydestruction contribute to anemia. With the exception of anemia caused by chronic renal failure or chronic inflammation (described
Table 12.1 Classification of Anemia According to Underlying Mechanism
Acute:traumaChronic:gastrointestinaltractlesions,gynecologicdisturbances
MembraneabnormalitiesMembraneskeletonproteins:spherocytosis,elliptocytosisMembranelipids:abetalipoproteinemia
EnzymedeficienciesEnzymesofhexosemonophosphateshunt:glucose-6-phosphatedehydrogenase,glutathionesynthetaseGlycolyticenzymes:pyruvatekinase,hexokinaseDisordersofhemoglobinsynthesisStructurallyabnormalglobinsynthesis(hemoglobinopathies):sicklecellanemia,unstablehemoglobinsDeficientglobinsynthesis:thalassemiasyndromes
Membranedefect:paroxysmalnocturnalhemoglobinuria