Lymph Node - Extramedullary Hematopoiesis

While the bone marrow is the major site of hematopoiesis, lymph nodes may be a site of extramedullary hematopoiesis (EMH). EMH in lymph nodes typically occurs as a physiologic response to a dramatic loss or increased need for additional blood cells. Examples of inciting factors include hemorrhage, infection, severe inflammation, myelofibrosis, and neoplasia. EMH is characterized by a mixture, to varying degrees, of myeloid, erythroid, and megakaryocytic cells and is primarily present within the lymph node medullary cords (Figure 1 and Figure 2). One lineage of hematopoietic cell may predominate, and various degrees of immaturity may be seen. EMH should be distinguished from inflammatory infiltrates and granulocytic leukemia. Inflammatory infiltrates in the lymph node are generally mature in development and may be a single or mixed cell type (see Lymph Node - Inflammation ). Granulocytic leukemia is characterized by high numbers of immature neoplastic myeloid cells that involve multiple organs. Previous terms for extramedullary hematopoiesis include ''hematopoietic cell proliferation" and "myeloid hyperplasia."