Leukemia is a type of cancer that starts in the blood cells, usually the bone marrow (the spongy tissue found inside the bones). The blood cells have three types of cells: the white blood cells that acts as an antigen against infection; the red blood cells that helps to circulate oxygen; and the platelets that help blood clots. Leukemia is the overproduction of abnormal white blood cells i.e. the part of the blood cells of the immune system which helps to defend the body against infection.

The white blood cells normally grow and divide itself in an orderly manner as the body grows either as additional stem cells or immature cells that become mature blood cell over time. But in people with leukemia, the bone marrow produces abnormal white blood cells that don’t function properly.

A blood stem cell may become a myeloid stem cell or a lymphoid stem cell.

A myeloid stem cell becomes one of three types of mature blood cells:

• Red blood cells that carry oxygen to all tissues of the body.
• Platelets that form blood clots to stop bleeding.
• Granulocytes (white blood cells) that fight infection and disease.

A lymphoid stem cell becomes a lymphoblast cell and then one of three types of lymphocytes (white blood cells):

• B lymphocytes that make antibodies to help fight infection.
• T lymphocytes that help B lymphocytes make the antibodies that help fight infection.
• Natural killer cells that attack cancer cells and viruses.

Many types of leukemia exists today, some forms of leukemia are more common in children while other forms of leukemia occur mostly in adults. Globally, the estimated number of people with Leukemia is at 749,565 (with over 300,000 new cases occurring after the 2018 statistics of 437,003 cases at 2.8% of new cancer cases), and mortality rate estimated at 400,021 cases.

How Leukemia Occurs

Leukemia is believed to occur when certain blood cells begin to develop various mutations in the human DNA, although there could be other changes in the blood cells that may contribute to the development of leukemia.

Certain abnormalities cause the cell to grow and divide more rapidly and to continue living when normal cells would die. Over time, these abnormal cells can crowd out healthy blood cells in the bone marrow, leading to fewer healthy white blood cells, red blood cells and platelets, causing the signs and symptoms of leukemia.

Classifications of Leukemia

Leukemia is usually classified into two parts: the first group is relating to how fast it develops and gets worse while the other group is by which type of blood cell is involved.

Leukemia by Development or Progress

Acute Leukemia: Here, most of the abnormal blood cells in the blood stem are usually immature blood cells called blasts; and can’t carry out normal functions required of them. Over time, these immature blood cells develop and multiply rapidly, making the disease grows worse quickly. Acute leukemia requires aggressive and well - timed treatment.

Chronic Leukemia: Chronic leukemia can develop various types of leukemia. Some of this chronic leukemia tends to produce too many cells, causing just a few blood cells to be produced. It usually involves more mature blood cells; reproducing more of these blood cells slowly and functioning normally for a period of time. As it progresses, it tends to get worse than acute leukemia. Also, chronic leukemia initially produces no clinical presentation and can go undiagnosed for a very long time.

Leukemia by White Blood Cells Involved

This group is divided into lymphocytic and myelogenous leukemia

• Lymphocytic Leukemia: This type of leukemia involves the bone marrow cell; affecting the lymphoid cells (lymphocytes) that usually form the lymphoid tissue. The lymphatic tissue is a type of white blood cell that makes up the immune system.
• Myelogenous Leukemia: This type of leukemia affects the myeloid cells that create the red blood cells, platelets, and other kinds of white blood producing cells.

Types of Leukemia

The major types of leukemia are:

• Acute Lymphocytic Leukemia (ALL): This is the most common type of leukemia that exists in children, although it also occur in adults. It can spread to the lymph nodes and central nervous system of the body causing malfunction to the central nervous system.
• Acute Myelogenous Leukemia (AML): This is the second most common type of leukemia that occurs mostly in children. It is one of the most common types of acute leukemia in adults.
• Chronic Lymphocytic Leukemia (CLL): This is another most common type of leukemia found in adults. Most types of CLL usually create stability in the human body, and creating no need for treatment for several years.
• Chronic Myelogenous Leukemia (CML): This type of leukemia affects adults mainly. People with CML do have unnoticeable or few symptoms for several months or years before the leukemia cells begin to grow more rapidly; and this is only diagnosed through a routine blood test. People who are aged 65 years and older have a higher risk at developing this type of leukemia.

There are other types of leukemia which are less common, and they include:

• Hairy Cell Leukemia: This is a rare, slow-growing cancer of the blood in which your bone marrow makes too many B cells (lymphocytes).
• Chronic Myelomonocytic Leukemia (CMML): This is another type of chronic leukemia that develops from myeloid cells. It starts in blood-forming cells of the bone marrow and invades the blood. It affects mainly older adults.
• Juvenile Myelomonocytic Leukemia (JMML): This is a type of myeloid leukemia that usually occurs in children under 6 years of age. It occurs when the types of white blood cells called monocytes and myelocytes do not mature normally.
• Large Granular Lymphocytic Leukemia (LGL leukemia): This is a type of chronic leukemia that develops from lymphoid cells. It can be slow or fast-growing.
• Acute Promyelocytic Leukemia (APL): This is a subtype of AML. It is an aggressive type of acute myeloid leukemia in which there are too many immature blood-forming cells (promyelocytes) in the blood and bone marrow.