Generally, cancers fall into one of two categories. They are either solid tumor or hematological (blood) cancers. Leukemia is a type of blood cancer that starts in the blood and bone marrow and can be categorized into four major types: acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL) and chronic myeloid leukemia (CML). These are described further below. Leukemias can also involve lymph nodes or the liver and spleen. An estimated 54,270 new cases of leukemia are expected to be diagnosed in the United States in 2015.
Leukemia is the abnormal growth and accumulation of blood cells in the body. The type of blood cells and how rapidly they grow denotes what type of leukemia a patient has. Unlike normal blood cells, these abnormal cells aren’t able to fight infections well and can inhibit normal body functions, including the ability to produce healthy blood cells. Patients usually have markedly low healthy white cells, red blood cells and platelets. Consequently they are at high risk of infection and need frequent red blood cell and platelet transfusions.
About blood and bone marrow
To fully understand leukemia, it’s important to first gain a general understanding of the components of blood and bone marrow (see Figure 1).
Bone marrow: The soft, spongy center of some bones that contains immature blood stem cells, mature blood-forming cells, fat cells, and tissues that support cell growth. This is the site where all blood cells are formed and mature before being released into the blood.
Platelets: A component of blood that groups around wounds to form clots and stop bleeding.
Red blood cells (erythrocytes): A component of blood responsible for delivering oxygen to the body.
White blood cells (leukocytes): A component of blood that helps the body defend against infection.
While there are several types of white blood cells, they can be divided into two main categories for the purpose of understanding leukemia:
Granulocytes are cells with enzyme-containing granules visible under a microscope. They develop from myeloblasts (immature cells found in bone marrow) into mature, infection-fighting cells. Subtypes of these cells include basophils, eosinophils and neutrophils.
Lymphocytes are the primary cells in lymphoid tissue, which is a major part of the immune system. They develop from lymphoblasts (immature cells found in bone marrow) into mature, infection-fighting cells. Subtypes of these cells include B lymphocytes and T lymphocytes.
Types of leukemia
The four major types of leukemia (ALL, AML, CLL, CML) can be divided into acute and chronic leukemia:
Acute leukemia grows quickly and occurs when immature white blood cells increase rapidly, preventing bone marrow from making normal blood cells. Treatment should be immediate because these fast-growing cells can quickly become life-threatening.
Chronic leukemia grows more slowly and occurs when white blood cells mature partly but not completely. These abnormal cells don’t fight infection as well as normal cells, and because they survive longer, they accumulate and crowd out normal cells. Progression is variable – some patients progress within weeks or months and some can be monitored safely for years.
Leukemias that begin in lymphoid stem cells are called lymphoid, lymphocytic or lymphoblastic leukemias; leukemias that start in myeloid cells are called myeloid, myelogenous, myelocytic or myeloblastic leukemias. See the following content for a full description of each of these types of leukemia.
In addition to these main types, a rare type of chronic leukemia, called hairy cell leukemia, affects lymphocytes in the bone marrow, blood and spleen. Under a microscope, these cells look like they’re covered with tiny hairs.
Risk factors for leukemia
While the exact cause of leukemia is unknown, certain factors can raise your risk, including exposure to high levels of radiation; exposure to high levels of certain chemicals; certain chemotherapy drugs; and some medical conditions, including inherited disorders involving chromosomal and blood abnormalities.
Symptoms of leukemia
Symptoms of leukemia vary among patients and depend on the number of leukemia cells in the bloodstream, the type of leukemia, and the area of the body where leukemia cells collect. People with chronic leukemia may have no symptoms or mild symptoms, while people with acute leukemia often feel sick.
If the brain is affected, symptoms may include vomiting, confusion, headaches, seizures or loss of muscle control. Leukemia may also affect the digestive tract, heart, lungs, kidneys or testes. Some other common symptoms of both acute and chronic leukemia include fever, chills, night sweats, swollen or tender lymph nodes, frequent and recurrent infections, weakness, fatigue, easy bruising and bleeding, unexplained weight loss, loss of appetite, discomfort or swelling in the stomach area, and bone or joint pain.