immature white blood cell (leukocyte) that gives rise to a type of immune cell known as a lymphocyte. The nucleus contains moderately fine chromatin (readily stainable nuclear material) and has a well-defined nuclear membrane. There are one or two nucleoli, and the cytoplasm is small or moderate in amount. Lymphoblasts that grow and divide uncontrollably cause a type of cancer known as acute lymphoblastic leukemia.