Bone marrow is spongy tissue found inside the bones. It is the source of cells called blasts, which mature into the different types of blood cells. White blood cells help fight infection; red blood cells carry oxygen and other nutrients to body tissue; platelets clot the blood.

The lymph nodes are small, bean-shaped organs of the immune system found in clusters in the abdomen, pelvis, underarms and neck. They are situated throughout the lymph system, a network of vessels throughout the body that carry lymph, a colorless fluid containing white blood cells.

AML (sometimes referred to as acute nonlymphocytic leukemia or acute myelogenous leukemia) develops in cells called myeloblasts, immature cells that eventually develop into mature white blood cells called granulocytes or monocytes.

In AML the bone marrow starts producing large numbers of abnormal blasts that overrun the bloodstream and lymph system. They can also invade the brain, skin, ovaries and testes, and other organs. AML cells occasionally form a solid tumor called an isolated granulocytic sarcoma or chloroma.

The subtypes of AML are based on the stage of development that myeloblasts have reached at the time of diagnosis.

If immature myeloblasts are the dominant cells at the time of diagnosis, the leukemia is referred to as "myeloblastic" type. If myeloblasts are starting to differ from one another, "differentiation" is added. If there are many myeloblasts, but some cells are beginning to develop into mature cells, the designation "with maturation" is added. If cells are developing features of monocytes, large, circulating white blood cells that have a single well-defined nucleus, the term monocytic is applied. Cells that are in the transition from myeloblasts to myelocytes (a large cell of the blood that eventually becomes a granulocyte, which has granules in its cytoplasm) are called promyelocytic. For those that are developing characteristics of red cells, erythroleukemia is used. Megakaryocytic leukemia cells may show a "budding." A numerical system is used to indicate these differences: