Leukemia is the name given to several types of cancer that begin in the tissue that forms blood. To understand leukemia, it helps to know how normal blood cells form.
Most blood cells develop in the marrow, the soft material at the center of most bones. Bone marrow contains hematopoietic stem cells, which can transform into different kinds of blood cells, depending on what kind your body needs at a particular time. When cells grow old or are damaged, they die, and hematopoietic stem cells replace them.
Each type of blood cell has a special job:
When someone develops leukemia, the bone marrow begins to make abnormal white blood cells, which are leukemia cells. Unlike normal blood cells, leukemia cells don't die when they should. They may crowd out normal white blood cells, red blood cells, and platelets. This makes it hard for normal blood cells to do their work.
The various types of leukemia are organized into two groups, based on how quickly the disease develops and gets worse, and the type of cells in which the cancer develops. Leukemia is either chronic (which usually gets worse slowly) or acute (which usually gets worse quickly).
The two types of leukemia can be further organized into groups that are based on the type of white blood cell that is affected — lymphoid or myeloid. Leukemia that affects lymphoid cells is called lymphoid, lymphocytic, or lymphoblastic leukemia. Leukemia that affects myeloid cells is called myeloid, myelogenous, or myeloblastic leukemia:
Early in the disease, the leukemia cells can still do some of the work of normal white blood cells. People may not have any symptoms at first. Doctors often discover chronic leukemia during a routine checkup, before there are any symptoms. Slowly, chronic leukemia gets worse. As the number of leukemia cells in the blood increases, people experience symptoms, such as swollen lymph nodes or infections. When symptoms do appear, they are usually mild at first but gradually get worse.
The leukemia cells can't do any of the work of normal white blood cells. The number of leukemia cells increases rapidly. Acute leukemia usually worsens quickly.
A few other, rarer types of leukemias, including hairy cell leukemia, account for more than 6,000 new leukemia diagnoses each year.
The symptoms of leukemia depend on the number of leukemia cells and where the cells collect in the body. They include: