Types of Leukemia: ALL, AML, CLL and CML

Different types of leukemia require different forms of treatment and affect different age groups. Survival rates often vary among the different types. To better understand how each variation of the cancer progresses, you need to know the differences between myeloid and lymphoid cells, and why the acute type spreads faster than chronic varieties.

Very immature cells, called blasts, characterize acute leukemia. Blasts reproduce quickly, crowding out healthy blood and bone marrow cells. Without treatment, acute forms of the disease are often fatal within months. Acute types of the cancer are the most common forms found in children.

 

Chronic varieties are more likely to be found in adults. In chronic conditions, blast cells are not the offending cells. Rather, the malignant cells have developed further, sometimes even resembling normal cells, although they are not able to function as well as healthy white blood cells (WBCs). Chronic forms of the disease progress more slowly.

There are two sets of WBC: myeloid and lymphoid. Lymphoid cells, or lymphocytic cells, are immune system cells related to B and T lymphocytes. WBCs of the myeloid type include granulocytes, which destroy bacteria, and monocytes, which fight infections and foreign substances. The disease is first classified depending on which of these two sets of cells become malignant, and further classified more specifically by the particular cell subtype.

AML develops from granulocyte precursor cells, which lead to an accumulation of immature blast cells and a reduction in healthy blood cells. As a result, the ability to fight infection is compromised, and AML often causes bleeding.

 

AML is further subdivided into eight sub-types. While most of these sub-types respond to the same therapies, some forms require different treatment approaches. The sub-types are classified by the types of WBCs involved, and the degree to which they mature.

While ALL can occur at any age, it is most common in children under the age of fifteen. Lymphocytes are unable to fully mature, and crowd out regular blood cells. ALL is subdivided into two types: B lymphocytic lineage and T lymphocytic lineage, depending on how closely the abnormal cells resemble "B" or "T" lymphocytes. 

CML affects immature granulocytes, halting their development while they are still in the blast stage. In addition, platelet precursor cells are sometimes also found in high levels. CML is easier to identify than other forms of the disease: over 95 percent of people with CML also test positive for the Philadelphia chromosome. The Philadelphia chromosome is formed by translocation, when genetic material from chromosome 9 swaps places with information on chromosome 22.

CLL is rarely diagnosed in persons under the age of fifty. Abnormal CLL lymphocytes resemble mature cells, and are very long-lived. Instead of rapid reproduction rates, it is the longevity of the cancer cells that slowly causes them to crowd out other blood cells. As a result, CLL is one of the slowest progressing forms of blood cancer.