Contributor: Gordon K. Klintworth
In  leukemia neoplastic leukocytes in the peripheral blood and the bone marrow is infiltrated with the tumor cells. The leukocytes may be derived from from the hematopoietic system (leukemia - myelogenous]) or lymphopoietic system lymphopoietic system (leukemia - lymphocytic]). Each of the cell types [stem cells (that give rise to granulocytes, monocytes,  erythrocytes, and megakaryocytes), cells of erythroid [erythroid cell], granulocytic [granulocyte], monocytic [monocyte] and megakaryocytic lineage can give rise to one or more neoplastic disorders. The neoplastic disorders vary with the cell of origin: stem cells (acute myelogenous leukemia [leukemia - acute myelogenous], granulocytes (chronic myelogenous leukemia [leukemia - chronic myelogenous], granulocytic leukemia [leukemia - granulocytic, monocytic leukemia [leukemia - monocytic], myelocytic leukemia [leukemia - myelocytic], myelomonocytic leukemia [leukemia - myelomonocytic]), erythroid cells (polycythemia vera), megakaryocytes [thrombocytopenia - idiopathic], Langerhans cells (Langerhans cell histiocytosis). All hematopoietic cell lines are involved in myelofibrosis with myeloid metaplasia.  Proliferative disorders of the lymphoid system may involve primarily the peripheral blood and bone marrow  (leukemia - acute lymphoblastic, leukemia - chronic lymphocytic). Individuals with a lymphoma may have an associated leukemia or the lymphoma may eventually evolve into a leukemia [leukemia/lymphoma - B-cell, leukemia/lymphoma - T-cell]. The bone marrow contains stem cells (that give rise to granulocytes, monocytes, erythrocytes and megakaryocytes) and cells of erythroid, granulocytic, monocytic and megakaryocytic lineage and each of these cell types can give rise to one or more neoplastic disorders. The neoplastic disorders vary with the cell of origin: stem cells (acute myelogenous leukemia [leukemia - acute myelogenous], granulocytes (chronic myelogenous leukemia [leukemia - chronic myelogenous], granulocytic leukemia [leukemia - granulocytic, monocytic leukemia [leukemia - monocytic], myelocytic leukemia [leukemia - myelocytic], myelomonocytic leukemia [leukemia - myelomonocytic]), erythroid cells (polycythemia vera), megakaryocytes [idiopathic thrombocytothemia], Langerhans cells (Langerhans histiocytosis). All hematopoietic cell lines are involved in myelofibrosis with myeloid metaplasia. Proliferative disorders of the lymphoid system may give rise to masses within the lymph nodes and other tissue sites (lymphoma or reactive lymphoid hyperplasia [lymphoid hyperplasia - reactive]. These proliferative disorders may reflect differentation and activation of  B-cells or T-cells.  They may also involve primarily the peripheral blood and bone marrow (acute lymphoblastic leukemia [leukemia - acute lymphoblastic], chronic lymphocytic leukemia [leukemia - chronic lymphocytic]). Individuals with a lymphoma may have an associated leukemia or the lymphoma may eventually evolve into a T-cell or B-cell leukemia.  The onset of leukemia may be acute [leukemia - acute] or insidious and chronic [leukemia - chronic]. Leukemia is malignant proliferation of leukocytes and is accompanied by the presence of neoplastic leukocytes in the peripheral blood. The bone marrow is infiltrated with the tumor cells. The leukocytes may be derived from from the hematopoietic system (myelogenous leukemia [leukemia - myelogenous]) or lymphopoietic system (lymphocytic leukemia [leukemia - lymphocytic]).