Cancer is a disease in which cells grow in an abnormal way. Normally, the cells divide in a controlled manner to replace old or damaged cells. With leukemia, blood cells begin to develop abnormally and rapidly divide creating more blood cells than needed. These cells develop in the bone marrow, but will eventually travel throughout the body in the bloodstream. The abnormal blood cells do not function as normal blood cells. They also crowd out normal cells, which can lead to problems with the immune system, oxygen delivery, and blood clotting.

All blood cells start as stem cells that are formed in bone marrow. Stem cells can mature into a variety of different blood cell types that have specific functions in the body. These include:

  • Red blood cells—Carry oxygen from the lungs to the organs and cells of the body.
  • Platelets—Stop bleeding by clotting broken blood vessels in a chain of complex chemical reactions.
  • White blood cells—Main line of defense for the body's immune system.

White Blood Cells

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New, healthy cells are developed in the bone marrow to replace old or damaged cells. This ensures there is a consistent number of blood cells in the body. With leukemia, there is an excessive development of new, abnormal blood cells. The abnormal cells crowd the bone marrow, making it difficult for new, healthy cells to develop. The abnormal cells and the lower levels of healthy cells lead to a weakened immune system, problems problems with oxygen delivery, or problems controlling bleeding depending on the type of blood cells that are affected. Cancerous blood cells also circulate in the blood and lymph systems, affecting organs like the spleen, liver, brain, and lymph nodes.

The instruction for cell growth and cell death exists in the DNA of each cell. Leukemia is the result of damage to DNA. The damage may be the result of genetics, environmental factors like radiation exposure, changes related to age, or a combination of these factors.

Cancer Cell Growth

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Leukemia is distinguished by how fast the leukemia develops and the type of blood cell that is affected. Leukemia may be distinguished as:

  • Acute—Abnormal cells are immature and the effect on the body is quickly apparent. Acute leukemias progress rapidly.
  • Chronic—Abnormal cells are mature and look similar to normal cells, but do not function as well. Chronic leukemia may go unnoticed for years until symptoms appear or it is found incidentally during a routine blood test.

Leukemia is also distinguished by the type of bone marrow cells that is starts in:

  • Myeloid—Affects myeloid cells which normally develop into certain white blood cells, red blood cells, and platelets.
  • Lymphocytic/lymphoblastic—Affects lymphoid cells that normally develop into a type of white blood cell called a lymphocyte.

Common types of leukemia include:

  • Acute lymphoblastic leukemia (ALL)—Most common type of leukemia in young children, but it can occur in older adults. ALL may also be referred to as acute lymphocytic leukemia or acute lymphoid anemia.
  • Acute myeloid leukemia (AML)—Occurs in both adults and children. May also be called acute myelocytic leukemia, acute myelogenous leukemia, acute granulocytic leukemia, or acute nonlymphocytic leukemia.
  • Chronic lymphocytic leukemia (CLL)— Occurs most often in older adults. Rarely, it can be found in younger adults and in children.
  • Chronic myeloid leukemia (CML)—Occurs mainly in adults, but may occur in a very small number of children. Sometimes called chronic myelogenous leukemia or chronic granulocytic leukemia.