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.
Leukemia develops when 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.
Read more: http://www.cancer.ca/en/cancer-information/cancer-type/leukemia/leukemia/?region=bc#ixzz3QeAiALzU