What Is Leukemia? Understanding the Main Types

The diagnosis depends on which blood cells are affected.

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The blood in your body seems simple and straightforward. It’s a red-hued liquid that pulses through your veins, delivering oxygen and other nutrients to tissues in your body. But in reality, blood is a complex fluid, and the number of cancers that can affect the blood exemplifies its complexity.

Leukemia falls under the vast blood cancer umbrella (which also includes lymphoma and myeloma). Leukemia is a type of blood cancer that affects blood cells in the bone marrow. “With leukemia, we usually have some kind of defect in a very early cell—the myeloid cells and the lymphoid cells,” says Michal Bar-Natan Zommer, MD, hematologist and oncologist at The Mount Sinai Hospital.

The bone marrow is the soft inner part of bones where new blood stem cells are created. In someone without leukemia, these stem cells slowly mature into the cells we know as red blood cells, white blood cells, and platelets, before circulating throughout the body as the blood you see when you donate blood or cut yourself in the kitchen.

But in someone with leukemia, cancer cells affect the blood stem cells in the bone marrow. Once cancerous, these cells no longer mature as they should, and they don’t die off as quickly as healthy blood cells do. The cancer cells do not function like healthy blood cells, thus depriving the individual of healthy blood function. Like all cancer cells, they grow and divide rapidly, crowding out the healthy cells in the bone marrow and potentially spreading to other parts of the body.

Reflecting the complexity of blood, you can divide leukemia into several subtypes depending on the rate of progression (acute or chronic) and the type of blood cell affected. The four most common types of leukemia are:

  • Acute myeloid leukemia (AML)
  • Chronic myeloid leukemia (CML)
  • Acute lymphocytic leukemia (ALL)
  • And chronic lymphocytic leukemia (CLL).

What Is Acute Myeloid Leukemia?

AML typically develops in the stem cells known as myeloid cells, which normally develop into mature red blood cells, platelets, or a type of white blood cell. There are three main types of white blood cells: granulocytes, monocytes, and lymphocytes. Cancers formed in stem cells that form lymphocytes are classified as ALL and CLL; cancers formed in stem cells that form other types of white blood cells are classified as AML or CML.

AML occurs from an acquired mutation—when the DNA of a myeloid stem cell is damaged—and the damaged cell multiplies into billions of leukemic cells known as leukemic blasts, according to the Leukemia and Lymphoma Society (LLS).

“Acute” refers to the progression of the disease. Acute leukemias can progress quickly and spread rapidly if untreated. It quickly spreads into the blood, and may spread to the lymph nodes, spleen, liver, and central nervous system, according to the American Cancer Society. “The majority of the patients can become symptomatic very early,” says Dr. Zommer.

What Is Chronic Myeloid Leukemia?

Like AML, chronic myeloid leukemia affects myeloid cells, an immature blood cell. The DNA change to the myeloid cell develops an abnormal gene known as BCR-ABL.

The difference is that CML does not totally disrupt the development of mature blood cells. The cells are able to mature partly, but not completely, according to the American Cancer Society. Leukemia cells in a chronic leukemia may look normal and retain some blood cell function, but they are not as effective as healthy mature blood cells. This makes CML less severe—but also harder to cure—than acute leukemias.

The slow-growing nature of chronic types of leukemia means that most patients have no symptoms for a long period of time. “The majority of the patients will be discovered as an accident [through] blood work that was done for something else,” says Dr. Zommer.

What Is Acute Lymphocytic Leukemia?

A lymphocytic leukemia affects an immature blood cell known as lymphocytes. Lymphocytes are a type of white blood cell that are found in blood and lymph tissue, and they support the immune system to fight off diseases.

In ALL, damage to the DNA of a developing stem cell creates a leukemic cell, which multiples rapidly into a leukemic lymphoblast. Because it’s acute, this type of leukemia progresses and spreads to other parts of the body (such as the central nervous system or lymph nodes) quickly if untreated.

What Is Chronic Lymphocytic Leukemia?

CLL is the most common type of leukemia in adults. Like the acute version, CLL begins in the lymphocytes (an immature white blood cell). However, because it’s chronic, this means the leukemia cells build up in the bone marrow slowly, and the individual may not have symptoms for years.

CLL is similar to a type of lymphoma known as small lymphocytic lymphoma (SLL). Both affect the lymphocytes. The difference is determined by where the cancer cells are primarily located, according to the Lymphoma Research Foundation. If the cancer cells are mostly located in the bone marrow, it is considered CLL (even if it has spread to the lymph nodes or spleen). If the cancer is primarily in the lymph nodes, the diagnosis is SLL.

Being diagnosed with any type of blood cancer can feel overwhelming, but treatment for blood cancer has become more effective. Here are important questions to ask your doctor after a blood cancer diagnosis, and find out self-care tips to cope with blood cancer treatment here.