Leukemia refers to a condition that occurs due to the rapid growth of abnormal white blood cells. In the US alone, this condition affects as many as 45,000 individuals annually. It is why this cancer is perhaps the most common blood cancer type, particularly in adults above 55 years of age and kids below 15 years of age.

To classify the types of leukemia, you have to understand the kind of blood cells that are affected, and the rate at which the condition progresses. For instance, lymphocytic leukemia, also known as lymphoblastic leukemia, originates in WBCs called lymphocytes. Myeloid leukemia may also originate in the lymphocytes, platelets, and red blood cells.

Depending on how quickly the condition advances, leukemia may be categorized as acute or chronic. Acute leukemia progresses rapidly and may result in the build-up of functionless and immature blood cells. In this type, cells reproduce and accumulate in the bone marrow and reduce its capacity to produce healthy blood cells. Chronic leukemia advances slowly, there is an accumulation of relatively mature yet abnormal WBCs. The 4 main types of leukemia include:

1. Acute Lymphocytic Leukemia (ALL)
ALL is known to progress rapidly. It can replace healthy cells with leukemia cells that do not mature properly. Such leukemia cells are then carried through the blood to other tissues and organs like the liver, brain, and lymph nodes where they grow and divide further. ALL is rare in grown-ups, but common in kids.

2. Acute Myeloid Leukemia (AML)
AML is one of the types of leukemia that occurs most often. AML is also called Acute Myeloblastic Leukemia, Acute Myelogenous Leukemia, Acute Nonlymphocytic Leukemia, and even Acute Granulocytic Leukemia. It is a fast-progressing cancer of the bone marrow and blood.

3. Chronic Lymphocytic Leukemia (CLL)
CLL is perhaps the most common of the types of leukemia in grown-ups. The therapies for this type are constantly evolving. It is a slow-growing cancer type that originates in lymphocytes inside the bone marrow and then extends into the blood. This cancer can even spread to the lymph nodes, as well as organs like the spleen and liver. CLL typically occurs when there is a growth of many abnormal lymphocytes that crowd out the normal blood cells and make it hard for the body to battle infections.

4. Chronic Myeloid Leukemia (CML)
CML is also called Chronic Myelogenous Leukemia. It originates in specific blood-forming cells in the bone marrow. Here, a genetic change happens in myeloid cells that affect the RBCs, platelets, and a majority of WBCs, except lymphocytes. The change is caused by a gene called BCR-ABL that converts a normal cell to a CML cell. The leukemia cells then continue to divide, accumulating inside the bone marrow and moving to the bloodstream. Slowly, these cells invade other body parts, including the spleen. It may be slow-growing but it can soon change into fast-growing cancer that is very hard to treat. This type is most prevalent in adults and rarely in kids.