Blast cells are immature precursors of either lymphocytes (lymphoblasts), or granulocytes (myeloblasts). They do not normally appear in peripheral blood. When they do, they can be recognized by their large size, and primitive nuclei (ie the nuclei contain nucleoli), as in the picture. When present in the blood, they often signify ACUTE LEUKEMIA. This particular case demonstrates the presence of an Auer Rod, which is pathognomonic for Acute Myeloid Leukemia. Otherwise, special stains and surface marker techniques are needed to identify the lineage of the cells.