Leukemia: Introduction

What is cancer?

Cancer is when cells in the body change and grow out of control. Your body is made up of tiny building blocks called cells. Normal cells grow when your body needs them, and die when your body does not need them any longer. Cancer is made up of abnormal cells that grow even though your body doesn’t need them. In most types of cancers, the abnormal cells grow to form a lump or mass called a tumor.

Leukemia is different from most other cancers. Leukemia is cancer that starts in the bone marrow. The bone marrow is where new blood cells are made. It's a thick, spongy liquid inside your bones.

Leukemia starts in early forms of blood cells, usually white blood cells, which help fight infections. When you have leukemia, your body makes too many blood cells, and they aren’t normal. Leukemia cells do not usually form tumors. But they can travel in the blood and go all over the body. That means they can reach almost any organ. So leukemia can cause problems and be found in many different ways, depending on which organs are involved.

Anatomy of  a bone, showing blood cells
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What are normal blood cells?

Blood is made up of liquid, called plasma, and 3 kinds of cells. Each kind of cell has a special task:

  1. White blood cells help the body fight infection and disease.

  2. Red blood cells carry oxygen from the lungs to the body's tissues and carry carbon dioxide from the tissues to the lungs.

  3. Platelets help form blood clots and control bleeding.

Blood cells are made in the soft center of the bones called the bone marrow. In adults, active bone marrow is found in the hip bones, ribs, spine, and skull. Normal cells in the bone marrow develop from very immature cells into mature, working cells ready to leave the bone marrow. Early, less mature, non-working forms of new blood cells are called blasts.

As cells mature in the bone marrow, they become smaller and more compact. They are better able to do their special jobs. Some new blood cells stay in the bone marrow to grow, while others move to other parts of the body to grow. Blood cells are produced at a higher rate when the body needs them, such as when a person has an infection or low numbers of red blood cells (anemia). This process helps the body stay healthy.

What are leukemia cells?

When a person has leukemia, the body makes too many blood cells of one type. These abnormal cells, usually white blood cells, don't mature and look different from normal blood cells. They don't work as they should. They also interfere with the making of other blood cells, usually red blood cells and platelets.

Types of leukemia

Two types of abnormal white blood cells can turn into leukemia: lymphoid cells and myeloid cells.

  • When leukemia involves the lymphoid cells, it is called lymphocytic or lymphoblastic leukemia. 

  • When it is found in the myeloid cells, it is called myelogenous or myeloid leukemia.

Leukemias are also grouped based on how fast they grow:

  • Acute leukemias get worse very quickly.

  • Chronic leukemias tend to be slow growing, but may get worse over time.

In acute leukemia, the abnormal blood cells are usually young cells (immature blasts) that do not work the way they should. These cells grow quickly. Acute leukemia quickly gets worse unless it is treated right away.

In chronic leukemia, young blood cells are present, but mature, functional cells are also made. In chronic leukemia, blasts grow slowly. It takes longer for the disease to get worse.

These categories result in 4 combinations, which make up the main types of leukemia:

  • Acute lymphoblastic leukemia (ALL). ALL is the most common type of leukemia in children. But it can occur in adults.

  • Acute myeloid leukemia (AML). AML is one of the most common types of acute leukemia in adults. It can also occur in children.

  • Chronic lymphocytic leukemia (CLL). CLL is also among the most common types of leukemia in adults. It is mostly seen in older adults. It may be seen in younger adults, but almost never in children.

  • Chronic myelogenous leukemia (CML). CML is a slightly less common type of leukemia that is seen mostly in adults. Very few children develop this type of leukemia.