Blast is a short name for an immature WBC, such as a lymphoblast or myeloblast. Normally, less than 5% of the cells in healthy bone marrow at any one time are blasts. While in the bone marrow, normal blasts develop into mature, functioning blood cells and are then released into the bloodstream. Therefore, in healthy people, blasts are not usually found in the bloodstream. Leukemic blasts remain immature, multiply continuously, provide no defense against infection, and may be present in large numbers in the blood and bone marrow.