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Childhood acute lymphoblastic leukemia (ALL) is a type of cancer in which the bone marrow makes too many lymphocytes (a type of white blood cell).

Childhood acute lymphoblastic leukemia (also called acute lymphocytic leukemia or ALL) is a cancer of the blood and bone marrow. This type of cancer usually gets worse quickly if it is not treated. It is the most common type of cancer in children.

Normally, the bone marrow makes blood stem cells (immature cells) that develop into mature blood cells over time. A blood stem cell may become a myeloid stem cell or a lymphoid stem cell.

The myeloid stem cell develops into one of three types of mature blood cells:

• Red blood cells that carry oxygen and other materials to all tissues of the body.



• Platelets that help prevent bleeding by causing blood clots to form.



• Granulocytes (white blood cells) that fight infection and disease.

The lymphoid stem cell develops into a lymphoblast cell and then into one of three types of lymphocytes (white blood cells):

• B lymphocytes that make antibodies to help fight infection.



• T lymphocytes that help B lymphocytes make the antibodies that help fight infection.



• Natural killer cells that attack cancer cells and viruses.

Blood cell development. A blood stem cell goes through several steps to become a red blood cell, platelet, or white blood cell.

In ALL, too many stem cells develop into lymphoblasts or lymphocytes. These cells may also be called leukemic cells. The leukemic cells are not able to fight infection very well. Also, as the number of lymphocytes increases in the blood and bone marrow, there is less room for healthy white blood cells, red blood cells, and platelets. This may lead to infection, anemia, and easy bleeding.

There are subgroups of childhood ALL.

Four of the subgroups of ALL are based on the type of blood cell that is affected, whether there are certain changes in the chromosomes, and age at diagnosis:

• T cell ALL.



• Philadelphia chromosome positive ALL.



• ALL diagnosed in an infant.



• ALL diagnosed in children who are aged 10 and older and adolescents (teenagers).

These subgroups are treated differently from other types of ALL.