Normal blood contains red blood cells, white blood cells and platelets. In leukemia, leukemia cells outnumber the normally seen healthy blood cells | Cleveland Clinic
Normal blood contains red blood cells, white blood cells and platelets. Leukemia cells outnumber normal cells in leukemia.

What is leukemia?

Leukemia is a cancer of the blood and bone marrow. In simple terms, cancer is defined as the uncontrolled growth of abnormal cells. Cancer can develop anywhere in the body. In leukemia, this rapid, out-of-control growth of abnormal cells takes place in the bone marrow of bones. These abnormal cells then spill into the bloodstream. Unlike other cancers, leukemia generally doesn’t form into a mass (tumor) that can be seen in imaging tests, such as X-rays.

There are many types of leukemia. Some are more common in children; others are more common in adults. Treatment depends on the type of leukemia you have and other factors.

What is bone marrow?

Bone marrow is the soft, spongy tissue in the center cavity of all bones. It is a limited space where all the different types of blood cells are made and where nutrients and other resources are supplied to help these cells grow. Blood cells keep our body healthy and functioning normally. More specifically, the different types of blood cells produced in the bone marrow include:

  • Red blood cells. These cells carry oxygen and other materials to all tissues and organs in the body.
  • White blood cells. These cells fight infection.
  • Platelets. Platelets help the blood clot.

Hundreds of billions of new blood cells are produced in the bone marrow each day, providing your body with a constant supply of fresh, healthy cells.

How does leukemia develop? How does leukemia affect the body?

Leukemia begins in the developing blood cells in the bone marrow. All blood cells start out as hematopoietic (hemo = blood; poiesis = make) stem cells. The stem cells undergo multiple stages of development until they reach their adult form.

First, blood stem cells develop into either myeloid cells or lymphoid cells. If blood cells were to continue to develop completely normally, the adult forms of these cells are as follows:

  • Myeloid cells develop into red blood cells, platelets, and certain types of white blood cells (basophils, eosinophils and neutrophils).
  • Lymphoid cells develop into certain types of white blood cells (lymphocytes and natural killer cells).

Process of how leukemia develops, starting in the bone marrow.

As stem cells in bone marrow begin to divide and multiply, they develop into all the needed types of blood cells. In patients with leukemia, cell growth goes "haywire," and there is a rapid growth of abnormal white blood cells.

So inside the bone marrow, blood cells are beginning to multiply and divide into red blood cells, white blood cells and platelets. However, if you have leukemia, one of these blood cell types begins to rapidly multiply, in an out-of-control manner. These abnormal cells – called leukemia cells – begin to take over the space inside the bone marrow. They crowd out the other normal cell types that are trying to develop. This is bad in a number of ways:

  • Unlike other blood cell types, the leukemia cells are abnormal and serve no useful purpose.
  • The other cell types (red blood cells, white blood cells and platelets) have very little space and support to continue to grow and multiply inside the bone marrow.
  • This results in fewer normal blood cells being made and released into the blood and more leukemia cells being made and released into the blood. Without an adequate amount of normal blood cells, your body’s organs and tissues will not get the oxygen they need to work properly, your body won’t be able to fight off infection or clot blood when needed.

Leukemia cells are usually immature (still developing) white blood cells. In fact, the term leukemia comes from the Greek words for "white" (leukos) and "blood" (haima). An excess number of white blood cells are seen when looking at blood through a microscope and the actual appearance of the blood is lighter to the naked eye.

Are there different types of leukemia?

Yes. Doctors classify leukemia by how quickly the disease worsens and by the type of blood cell involved.

By speed of disease development:

  • Acute leukemia. The leukemia cells are rapidly dividing and the disease progresses quickly. If you have an acute leukemia, you would feel sick within weeks of the leukemia cells forming. Acute leukemia is the most common pediatric cancer.
  • Chronic leukemia. Often, these leukemia cells have features of both immature and mature cells. Some of these cells may have developed to the point where they do function as the cells they were meant to become, but not to the extent their normal counterparts do. The disease typically worsens slowly as compared to acute leukemia. If you have chronic leukemia, you may not have noticeable symptoms for years. Chronic leukemia is more commonly seen in adults as compared to children.

By cell type:

  • Myelogenous or myeloid leukemia means the leukemia has developed from the myeloid cell line. Normal myeloid cells develop into red blood cells, white blood cells and platelets.
  • Lymphocytic leukemia means the leukemia has developed from the lymphoid cell line. Normal lymphoid cells develop into white blood cells that are an important part of the body’s immune system.

There are four major types of leukemia:

  • Acute myeloid leukemia (AML): This is the most common type of acute leukemia. It is more common in older adults (those over 65 years of age) and in men compared with women. About 4.3 per 100,000 men and women or 21,400 new cases of AML per year are diagnosed in the United States.
  • Acute lymphocytic leukemia (ALL): This is the most common type of leukemia in children, teens, young adults and those up to 39 years of age. About 54% of new cases occur in those under the age of 20. It is more common in persons of Hispanic and White origin. About 1.7 per 100,000 men and women or 5,900 new cases of ALL per year are diagnosed in the United States.
  • Chronic myelogenous leukemia (CML): This leukemia is more common in older adults (most common in those over 65 years of age) and in men. It rarely occurs in children. About 1.9 per 100,000 men and women or 8,900 new cases of CML per year are diagnosed in the United States.
  • Chronic lymphocytic leukemia (CLL): This is the most common chronic leukemia in adults (most common in those over 65 years of age). It is more common in men than women and especially in white men. About 4.9 per 100,000 men and women or 20,700 new cases of CLL per year are diagnosed in the United States.

In addition to these four main types of leukemia, there also are various subtypes of leukemia. Subtypes of lymphocytic leukemia include hairy cell, Waldenstrom’s macroglobulinemia, prolymphocytic, and lymphoma cell leukemia. Among the subtypes of myelogenous leukemia are myelogenous, promyelocytic, monocytic, erythroleukemia, and megakaryocytic leukemia.

How common is leukemia?

The number of new cases of leukemia diagnosed in the United States each year is about 14 per 100,000 men and women or 61,000 new cases per year. It is the tenth most common cancer according to new cases diagnosed each year. Leukemia accounts for 3.5% of all new cancer cases in the United States.

Leukemia is often considered a disease of children, yet it actually affects far more adults. In fact, the likelihood of developing this cancer increases with age. Leukemia is most frequently diagnosed in people 65 to 74 years of age. Leukemia is more common in men than in women, and more common in Caucasians than in African-Americans. Although leukemia is rare in children, of the children or teens who develop any type of cancer, 30% will develop some form of leukemia.

What causes leukemia?

Leukemia starts when the DNA of a single cell in the bone marrow changes (mutates) and can’t develop and function normally. (DNA is the “instruction code” for the cell’s growth and function. Segments of DNA make up genes, which are arranged on larger structures called chromosomes.) All cells that arise from that initial mutated cell also have the mutated DNA.

What causes the damage to the DNA in the first place is still not known in all cases. Scientists have been able to locate changes in certain chromosomes of patients diagnosed with different types of leukemia.

Who gets leukemia? Are certain people at higher risk for developing leukemia?

Although the exact cause of the DNA mutation that leads to leukemia is not fully known, scientists have discovered certain risk factors that may increase your risk of developing leukemia. These risk factors include:

  • Previous cancer treatment with radiation or chemotherapy.
  • History of smoking or working with industrial chemicals. Benzene and formaldehyde are known cancer-causing chemicals found in tobacco smoke and building materials and household chemicals. Benzene is used in the making of plastics, rubbers, dyes, pesticides, drugs and detergents. Formaldehyde is found in building materials and many household products such as soaps, shampoos and cleaning products.
  • Having a genetic disorder, such as neurofibromatosis, Klinefelter syndrome, Schwachman-Diamond syndrome or Down Syndrome.

Leukemia can happen to anyone. You may get leukemia and have none of these risk factors. Other people have one or more of these risk factors and never get leukemia.

You cannot “catch” leukemia from someone else. It is not “transmitted” from one person to another.

Does leukemia run in families? Can leukemia be inherited?

Yes, however this is uncommon. Genetic disorders such as Down syndrome can increase the risk of leukemia. Scientists have also found other genetic mutations that can increase the risk. How much the risk is increased is not exactly known. Having a relative in your family with leukemia does not mean you or your family members will also develop leukemia. In fact, in most cases, there’s no family history of leukemia. However, if you or a family member has a genetic condition, tell your doctor. Your doctor may recommend genetic testing or counseling.

What are the symptoms of leukemia?

Your symptoms depend, in part, on what type of leukemia you have. However, common signs and symptoms include:

  • Tire easily, little energy, weakness.
  • Pale skin tone.
  • Fever.
  • Easy bruising and bleeding. Nosebleeds and bleeding gums. Tiny red spots in skin (called petechiae). Purplish patches in the skin.
  • Bone or joint pain and/or tenderness.
  • Swollen lymph nodes in the neck, underarm, groin or stomach; enlarged spleen or liver.
  • Frequent infections.
  • Unplanned weight loss.
  • Night sweats.
  • Shortness of breath.
  • Pain or full feeling under the ribs on the left side.

Keep in mind that if you have a chronic form of leukemia, you may not have any noticeable symptoms in the early stages of this cancer.

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