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.
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. Lymphocytic leukemias (also known as lymphoblastic leukemias) 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.
The 4 main types of leukemia are acute lymphocytic leukemia (ALL), acute myelogenous leukemia (AML), chronic lymphocytic leukemia (CLL) and chronic myelogenous leukemia (CML).
In adults, CLL and AML are the most common leukemias. There are many different subtypes of leukemia.