Hematopathology is the branch of pathology which studies diseases of hematopoietic cells (see below). In the United States, hematopathology is a board certified subspecialty (American Board of Pathology) practiced by those physicians who have completed general pathology residency (anatomic, clinical, or combined) and additional fellowship training in hematology.
Hematopoietic cells originate from the bone marrow and contribute the cellular components of blood including red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (megakaryocyte-derived bodies). Additionally, leukocytes contribute significantly to the cellular composition of lymph nodes, the spleen, the thymus, and mucosa associated lymphoid tissues, and are present to some degree in all tissues. Diseases of the hematopoietic system generally manifest as decreases (anemia) or increases (lymphoma/leukemia) in these cell types. Consequently, hematopathologists most frequently evaluate peripheral blood smears, bone marrow aspirates and biopsies, and lymph nodes biopsies to determine the nature of hematopoietic disease. The hematopathologist incorporates traditional microscopy with ancillary techniques including general laboratory values, immunohistochemistry, flow cytometry, and molecular diagnostic tests to make the most accurate diagnosis. The hematopathologist works closely with the hematologist/oncologist specialty doctor who sees the patient and decides on the best treatment based upon the diagnosis made by the hematopathologist.
Diseases of Hematopoietic Cells 
Diseases of the hematopoietic system are myriad and include hereditary and congenital disorders as well as acquired disorders. Hereditary and congenital disorders include, in part, bone marrow failure syndromes and primary deficiency syndromes. Acquired disorders may be related to nutritional deficiencies (such as iron, folate, and B12 deficient anemias), infectious processes or neoplastic disorders (cancers). Diagnostic criteria and classification guidelines for the neoplastic diseases are codified in the WHO Classification of Tumours of the Hematopoietic and Lymphoid Tissue, most recently revised in 2008. The neoplastic diseases of hematopoietic cells fall into two broad categories, myeloid neoplasms and lymphoid neoplasms. Histiocytic and dendritic cell neoplasms are considered separately.
– Myeloid Neoplasms:
Myeloproliferative neoplasms Myeloid and lymphoid disorders with eosinophilia Myeloproliferative/myelodysplastic neoplasms Myelodysplastic syndromes Acute myeloid leukemia and related precursor neoplasms Acute leukemia of ambiguous lineage
– Lymphoid Neoplasms
Precursor lymphoid neoplasms Mature B-cell neoplasms Mature T-cell neoplasms Hodgkin lymphoma Immunodeficiency associated lymphoproliferative disorders
– Histiocytic and Dendritic Cell Neoplasms
Each category contains several recognized diagnostic entities resulting in nearly 100 different types of hematopoietic cancer, each with defining morphology, pathobiology, treatment, and/or prognostic features. Correct classification, as well as identification of additional factors that may influence prognosis or response to chemotherapy, is essential to allow optimal treatment by the hematologist/oncologist. The hematopathologist is also responsible for additional studies performed after initiation of therapy to monitor for continued remission or disease relapse.