Leukemia

What is leukemia?

Leukemia is cancer that starts in the tissue that forms blood.
To understand cancer, it helps to know how normal blood cells form.

Normal Blood Cells

Most blood cells develop from cells in the bone marrow called stem cell.
Stem cells mature into different kinds of blood cells. 

• White blood cells, red blood cells, and platelets

When cells grow old or get damaged, they die, and new cells take their place.

First, a stem cell matures into either a myeloid stem cell or a lymphoid stem cell:

• A myeloid stem cell matures into a myeloid blast. The blast can form a red blood cell, platelets, or one of several types of white blood cells.
• A lymphoid stem cell matures into a lymphoid blast. The blast can form one of several types of white blood cells, such as B cells or T cells.

The white blood cells that form from myeloid blasts are different from the white blood cells that form from lymphoid blasts.

Leukemia Cells

In a person with leukemia, the bone marrow makes abnormal white blood cells.
The abnormal cells are leukemia cells.
Unlike normal blood cells, leukemia cells don't die when they should.
They may crowd out normal white blood cells, red blood cells, and platelets.
This makes it hard for normal blood cells to do their work.

Types of Leukemia

Leukemia is either chronic (which usually gets worse slowly) or
acute (which usually gets worse quickly):

• Chronic leukemia: Early in the disease, leukemia person may not have any symptoms at first. As the number of leukemia cells in the blood increases, people get symptoms, such as swollen lymph nodes or infections.
  
• Acute leukemia: The leukemia cells can't do any of the work of normal white blood cells. The number of leukemia cells increases rapidly.

The types of leukemia also can be grouped based on the type of white blood cell that is affected.
Leukemia can start in lymphoid cells or myeloid cells.
Leukemia that affects lymphoid cells is called lymphoid, lymphocytic, or lymphoblastic leukemia.
Leukemia that affects myeloid cells is called myeloid, myelogenous, or myeloblastic leukemia.
There are four common types of leukemia:

• Chronic lymphocytic leukemia (CLL): CLL affects lymphoid cells and usually grows slowly. Most often, people diagnosed with the disease are over age 55. 

• Chronic myeloid leukemia (CML): CML affects myeloid cells and usually grows slowly at first. It mainly affects adults.

• Acute lymphocytic (lymphoblastic) leukemia (ALL): ALL affects lymphoid cells and grows quickly.  ALL is the most common type of leukemia in young children. It also affects adults.

• Acute myeloid leukemia (AML): AML affects myeloid cells and grows quickly.  It occurs in both adults and children.

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