What is leukemia?

Leukemia is cancer that starts in the tissue that forms blood. To understand cancer, it helps to know how normal blood cells form.


Normal 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.

Stem cells mature into different kinds of blood cells. Each kind has a special job:

White blood cells help fight infection. There are several types of white blood cells.

Red blood cells carry oxygen to tissues throughout the body.

Platelets help form blood clots that control bleeding.

White blood cells, red blood cells, and platelets are made from stem cells as the body needs them. When cells grow old or get damaged, they die, and new cells take their place.

The picture below shows how stem cells can mature into different types of white blood cells. First, a stem cell matures into either a myeloid stem cell or a lymphoid stem cell:

•     A myeloid stem cell matures into a myeloid blast. The blast can form a red blood cell, platelets, or one of several types of white blood cells.

•     A lymphoid stem cell matures into a lymphoid blast. The blast can form one of several types of white blood cells, such as B cells or T cells.

The white blood cells that form from myeloid blasts are different from the white blood cells that form from lymphoid blasts.

Leukemia Cells

In a person with leukemia, the bone marrow makes abnormal white blood cells. The abnormal cells 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.