What is acute lymphoblastic leukemia?

Acute lymphoblastic leukemia (ALL) is a cancer that starts in stem cells of the blood. Stem cells are basic cells that develop into different types of cells that have different jobs. As the stem cells of the blood develop, they become blast cells (blasts), which are immature blood cells. In leukemia, there is an overproduction of blast cells. These blast cells develop abnormally and don’t develop into mature blood cells. Over time, the blast cells crowd out normal blood cells so that they can’t do their jobs. When leukemia is diagnosed, these blast cells may be called leukemia cells.

There are many different types of leukemia. They are grouped based on the type of blood stem cell they developed from. Blood stem cells develop into either lymphoid stem cells or myeloid stem cells.

Lymphoblastic leukemias (also known as lymphocytic leukemias) develop from abnormal lymphoid stem cells. Lymphoid stem cells normally develop into lymphocytes, a type of white blood cell. Lymphocytes are found in blood and different parts of the lymphatic system, such as the lymph nodes and spleen. The 3 types of lymphocytes are B cells, T cells and natural killer (NK) cells. Lymphocytes help fight infection and destroy abnormal cells.

The types of leukemia are further grouped based on how quickly the leukemia develops and grows. Acute leukemias start suddenly, developing within days or weeks. Chronic leukemias usually develop slowly over months or years.

ALL starts in abnormal lymphoid stem cells and develops quickly. ALL is the most common type of leukemia diagnosed in young children, and it occurs more often in boys than girls. It is the least common of the 4 major types of leukemia in adults. In about 25% of adults with ALL, the leukemia cells have the Philadelphia (Ph) chromosome. This is an acquired chromosomal abnormality, which means that is occurs some time after birth.