Blood is a vital liquid that carries oxygen, nutrients, hormones, and other necessary chemicals to the body’s cells. It also removes toxins and other waste products from the cells. Blood cells carry oxygen, fight infections, and help repair injuries by slowing or stopping bleeding.

Whole blood is made up of plasma—a clear fluid—and many other components, each with a specific task. Blood contains three types of blood cells—red blood cells, white blood cells, and platelets. Red blood cells (also called erythrocytes or RBCs) contain hemoglobin, a protein that carries oxygen from the lungs to other cells in the body. RBCs carrying oxygen give blood its red color. When leukemia cells in the bone marrow interfere with production of RBCs, anemia develops. Anemia can cause tiredness, weakness, irritability, pale skin, and headaches—all due to not enough oxygen being carried to the body tissues.

Healthy white blood cells (also called leukocytes or WBCs) destroy foreign substances in the body, such as viruses, bacteria, and fungi. WBCs are produced and stored in the bone marrow and lymph nodes. They are released when needed by the body. If an infection is present, the body produces extra WBCs. There are two main types of WBCs:

• Lymphocytes: The three types of lymphocytes are T cells, B cells, and natural killer cells.
• Granulocytes: The four types of granulocytes are neutrophils, monocytes, eosinophils, and basophils.

Platelets are tiny, disc-shaped cells that help form clots to stop bleeding. Leukemia interferes with production of platelets, causing a dramatic drop in platelets in children with the disease, which can lead to excessive bleeding from the nose and gums or from cuts. Children with leukemia can develop large bruises or small red dots (petechiae) on their skin or in the mouth and throat.

Our daughter Emi, who has Down syndrome, was diagnosed with pneumonia at 14 months. Her blood counts showed a very low platelet count and a low white blood cell count. She was treated for the pneumonia, and when we went back for more blood work a few weeks later, her counts were even lower and the doctors thought they saw some blasts [immature white blood cells]. They sent the results to a hem-onc doctor who scheduled a bone marrow aspiration a few days later. They said they saw leukemia cells in the bone marrow, and called the next day to say it was AML. We went straight to the children’s hospital and spent most of the next six months there.

Normally, the bone marrow makes blood stem cells (immature cells) that become mature blood cells over time. A blood stem cell develops into either a myeloid stem cell or a lymphoid stem cell (see Figure 2–1). A myeloid stem cell becomes a WBC, a RBC, or a platelet. A lymphoid stem cell becomes a WBC.

Figure 2–1: Development of mature blood cells from stem cells

The different types of leukemia are cancers of a specific WBC type. For instance, acute lymphoblastic leukemia is a cancer of the lymphocytes, and acute myeloid leukemia is cancer of the granulocytes.