Image: Cancerous lymphoid cells from a patient with Acute lymphocytic leukemia (ALL).

The first step in maturation of blood-forming cells in the bone marrow is differentiation into 2 groups: the myeloid cells and the lymphoid cells.

• The myeloid cells develop into red blood cells, platelets, and certain types of white blood cells (granulocytes or monocytes).
• The lymphoid cells develop into another type of white blood cell (lymphocytes).

Either lineage can be affected by leukemia, a cancer of these cells. Cancerous cells grow rapidly but do not fully mature and are unable to carry out the functions of healthy cells.

Leukemias that affect lymphoid cells are called lymphocytic leukemias. Acute leukemias are those of rapid onset. Acute lymphocytic leukemia (ALL) affects both children and adults but is more common in children. It accounts for 65% of acute leukemias in children. Untreated, death occurs within weeks or a few months.