Very early on in hematopoiesis, the stem cell chooses its path of development or "cell line." One cell line, the lymphoid, ultimately results in B lymphocytes and T lymphocytes. The other cell line, the myeloid, leads to the development of platelets, red blood cells, neutrophils and other types of white blood cells. The term "blast cells" refers to myeloblasts or myeloid blasts. These are the very earliest and most immature cells of the myeloid cell line.
Although the cells of the myeloid cell line make up about 85% of the cells in bone marrow, less than 5% should be blast cells. Blast cells are not typically found in the circulating blood of healthy individuals.
In the case of acute myelogenous leukemia (AML) and myelodysplastic syndromes (MDS), there is an overproduction of abnormal myeloblasts. These cells are unable to develop further into mature white blood cells.
These abnormal blasts begin to take over the bone marrow and prevent production of adequate numbers of other types of blood cells, such as platelets, red blood cells and healthy white blood cells. In fact, production of leukemic blasts may get so out of hand, that the immature cells spill out from the bone marrow into the circulation. The presence of blast cells on a complete blood count (CBC) is therefore very suspicious of leukemia
Williams, L. "Comprehensive Review of Hematopoiesis and Immunology: Implications for Hematopoietic Stem Cell Transplant Recipients" in Ezzone, S. (2004) Hematopoietic Stem Cell Transplantation: A Manual for Nursing Practice Oncology Nursing Society: Pittsburgh, PA (pp.1- 12)