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Leukemia is a cancer that starts in blood stem cells. Stem cells are basic cells that develop into different types of cells that have different jobs.
Blood stem cells develop into either lymphoid stem cells or myeloid stem cells.
Leukemia develops when the blood stem cells in the bone marrow change and no longer grow or behave normally. These abnormal cells are called leukemia cells. Over time, the leukemia cells crowd out normal blood cells so that they can’t do their jobs.
There are many different types of leukemia. They are grouped based on the type of blood stem cell they developed from. Lymphoblastic leukemias (usually known as lymphocytic leukemias in adults) develop from abnormal lymphoid stem cells. Myelogenous leukemias develop from abnormal myeloid stem 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 develop slowly over months or years.
Acute lymphoblastic leukemia (ALL) is the most common type of leukemia diagnosed in young children, and it occurs more often in boys than girls. Acute myelogenous leukemia (AML) is less common and usually occurs more often in girls than boys.
Rare types of childhood leukemias can also develop. These include chronic lymphoblastic leukemia (CLL) and chronic myelogenous leukemia (CML).