Leukemia is cancer of the white blood cells. Under normal circumstances, white blood cells help the body fight infection. In leukemia, however, the bone marrow (the spongy tissue inside bones where blood cells originate) produces abnormal white blood cells. These cells crowd out healthy blood cells, making it difficult for blood to do its work.

Types of leukemia are named after the specific blood cell that becomes cancerous. The four main types of leukemia include:

  • Acute lymphocytic leukemia (ALL) -  This is a type of cancer in which the bone marrow produces too many lymphocytes (a type of white blood cell). This type of leukemia occurs most often in children.
  • Chronic lymphocytic leukemia (CLL) CLL is a type of cancer of the blood and bone marrow that is most common in older adults.
  •  Acute myeloid leukemia (AML) Also known as acute myelogenous leukemia, this cancer is characterized by the rapid growth of abnormal white blood cells that accumulate in the bone marrow and interfere with the production of normal blooc cells. The most common type of leukemia  affecting adults, the incidence of AML increases with age.
  • Chronic myeloid leukemia (CML) - Also known as chronic granulocytic leukemia (CGL), CML is a chronic, slow-growing cancer of the white blood cells. 

There are other, less common types of leukemia but they are generally sub-categories of one of the four main categories. According to the National Cancer Institute, more than 40,800 adults and 4,500 Americans are diagnosed with this disease each year.