In order to understand the different types of leukemia, it helps to have some basic knowledge of the blood and lymph systems.

Bone Marrow

Bone marrow is the soft, spongy, inner part of bones. All of the different types of blood cells are made in the bone marrow. Bone marrow is made up of blood stem cells, blood-forming cells, fat cells, and tissues that aid the growth of blood cells.

The blood-forming cells come from blood stem cells. They only make new blood-forming cells and not other kinds of cells. (They are different from embryonic stem cells which are formed from a developing fetus and can grow to become other kinds of cells in the body.) The blood-forming cells can develop into 1 of the 3 main types of blood cell: red blood cells, white blood cells, or platelets.

Red Blood Cells

Red blood cells carry oxygen from the lungs to all other tissues of the body. They also carry away carbon dioxide, a waste product of cell activity. A shortage of red blood cells causes weakness, shortness of breath, and tiredness.

Platelets

Platelets are actually pieces that break off from certain bone marrow cells. Platelets help stop bleeding by plugging up holes in blood vessels caused by cuts or bruises. A shortage of platelets can cause a person to bleed or bruise easily.

White Blood Cells

White blood cells help defend the body against germs such as viruses and bacteria. There are quite a few types of white blood cells. Each has a special role to play in protecting the body against infection. The 3 main types of white blood cells are granulocytes, monocytes, and lymphocytes.

Any of the blood-forming cells can turn into a leukemic cell. Once that happens, the cell can reproduce to form many new cancer cells. These cells can overwhelm the bone marrow, spill out into the bloodstream, and spread to other organs.