This term refers to the earliest marrow cells identified by the light microscope. Blasts represent about 1 percent of normally developing marrow cells. They are largely myeloblasts, which are cells that will develop into neutrophils. In normal lymph nodes, blasts are lymphoblasts, that is, cells that are part of lymphocyte development. In the acute leukemias, blast cells, similar in appearance to normal blast cells, accumulate in large numbers, perhaps up to 80 percent of all marrow cells. In acute myelogenous leukemia, myeloblasts accumulate and in acute lymphoblastic leukemia, lymphoblasts accumulate. Sometimes the distinction between myeloblasts and lymphoblasts can be made by examination of stained marrow cells through the microscope. Often, immunophenotyping or use of special staining of marrow cells is required to be sure of the distinction.

Immature cells that mature into the three types of blood cells.

Immature white blood cells. Healthy bone marrow makes stem cells that grow into red blood cells, white blood cells, and platelets. A leukemia patient's bone marrow makes too many blast cells (immature white blood cells). Leukemic blast cells remain immature and do not function like mature white blood cells, which are supposed to destroy infection by bacteria and fungi. This results in anemia and vulnerability to infection.