Leukemia is a cancer of the blood or bone marrow that starts in the tissue that produces blood cells. Most blood cells develop from cells in the bone marrow called stem cells. Bone marrow is the soft material in the center of most bones.

When you are healthy, your bone marrow makes:

  • White blood cells, which help your body fight infection
  • Red blood cells, which carry oxygen to all parts of your body
  • Platelets, which help your blood clot

When you have leukemia, your bone marrow makes a lot of abnormal white blood cells, called leukemia cells. These abnormal cells don’t do the work of normal white blood cells, they grow faster than normal cells, and they don’t stop growing when they should.

Over time, leukemia cells can crowd out normal blood cells. This can lead to serious problems such as anemia, bleeding, and infections. Leukemia cells can also spread to the lymph nodes or other organs and cause swelling, pain, or death.

There are four common types of adult leukemia. Two are chronic (chronic lymphocytic leukemia and chronic myeloid leukemia), meaning they get worse over a longer period of time. And two are acute (acute myeloid leukemia and acute lymphocytic leukemia), meaning they get worse quickly.

Chronic lymphocytic leukemia, chronic myeloid leukemia, and acute myeloid leukemia are diagnosed more often in older adults. Of these, chronic lymphocytic leukemia is the most common. Acute lymphocytic leukemia is found more often in children.