Cancer is a process of uncontrolled abnormal cell growth and development. Under normal circumstances, cells are formed, mature, carry out their intended function, and then die. New cells are constantly regenerated in the body to replace those cells and to maintain normal cellular function.
Cancer represents the disturbance of this process, which can occur in several ways. Cells may grow and reproduce in a disorganized and out-of-control fashion. Cells may fail to develop properly, so they will not function normally. Cells may fail to die normally. One or a combination of these processes may occur when cells become cancerous.
Leukemia is a cancer of blood-forming cells in the bone marrow. These deranged, immature cells accumulate in the blood and within organs of the body. They are not able to carry out the normal functions of blood cells.
Normal blood contains 3 major groups of cells: white blood cells, red blood cells, and platelets. All 3 types of blood cells develop from one immature cell type, called blood/marrow stem cells, in a process called hematopoiesis.
These stem cells divide and develop to a more developed, but still immature precursor, called a blast, which then develops through several more stages, into a mature blood cell. This process takes place in the bone marrow, which is the soft spongy material found in the center of most bones.
Each type of blood cells has its own different and essential function in the body.
- White blood cells (leukocytes) are part of the immune system and help fight a variety of infections. They also help in the healing of wounds, cuts, and sores.
- Red blood cells (erythrocytes) contain hemoglobin, which carries oxygen to, and removes carbon dioxide from, the cells throughout the various organs of the body.
- Platelets, along with certain plasma proteins, help plug the holes in blood vessels and form clots once blood vessels are damaged or cut.
The first step in the process of stem cell maturation is differentiation into 2 groups: the myeloid stem cell line and the lymphoid stem cell line.
- The myeloid stem cells, or lineage, develop into red blood cells, platelets, and certain types of white blood cells (granulocytes or monocytes).
- The lymphoid stem cells, or lineage, develop into another type of white blood cell (lymphocytes).
Either lineage can be affected by leukemia. Leukemias that affect the myeloid lineage are called myelocytic (also myelogenous, myeloblastic, or nonlymphocytic) leukemias. Leukemias that affect the lymphoid lineage are called lymphocytic (also lymphoblastic or lymphogenous) leukemias.
Each of the two major types of leukemia, myelogenous and lymphocytic, include both acute and chronic forms.
- Acute essentially refers to a disorder of rapid onset. In the acute myelocytic leukemias, the abnormal cells grow rapidly and do not mature. Most of these immature cells tend to die rapidly. In the acute lymphocytic leukemias, growth is not as rapid as that of the myelocytic cells. Rather, the cells tend to accumulate. Common to both types of leukemia is their inability to carry out the functions of healthy white blood cells. Untreated, death occurs within weeks or a few months.
- In the chronic leukemias, the onset tends to be slow, and the cells generally mature abnormally and often accumulate in various organs, often over long intervals. Their ability to fight infections and assist in repairing injured tissues is impaired. However, unlike the acute forms of leukemia, untreated, these disorders may persist for many months or, as in the chronic lymphocytic group, many years. A distinctive feature of the chronic myelocytic type is its invariable conversion, if untreated, to a more rapidly fulminating acute type, leading to rapid death.