There are three types of myeloproliferative neoplasms: polycythemia vera (PV), the most common type; myelofibrosis (MF), the most aggressive type; and essential thrombocythemia (ET), the most indolent type.

Polycythemia vera occurs when there is an elevation of the red blood cells. The disease can result in elevated white blood cell count, an elevated platelet count and an enlarged spleen over time.

Myelofibrosis occurs when excessive scar tissue forms in the bone marrow and impairs its ability to produce normal blood cells. The abnormal stem cells in myelofibrosis produce mature cells that grow quickly and take over the bone marrow, causing fibrosis and chronic inflammation that results in difficulty for the bone marrow to create normal blood cells. In turn, blood cell production may move to the spleen, causing enlargement, or to other areas of the body. Myelofibrosis can develop on its own or as progression of polycythemia vera or essential thrombocythemia.

Essential thrombocythemia occurs when there is an increased number of platelets in the blood and it is associated with a proliferation of platelet precursors in the bone marrow. Blood clots and/or bleeding can often occur with essential thrombocythemia.