Bone marrow is a spongy tissue found inside our bones. There are two types of bone marrow found in our bones – red and yellow bone marrow.

Red marrow produces blood cells, which consist of:

• Red blood cells – They carry the blood to all parts of the body.
• White blood cells – They fight against infections.
• Platelets – They are the clotting agents that protect our body from uncontrolled bleeding.

Yellow marrow stores our fat cells called adipocytes. Yellow marrow also contains mesenchymal stem cells, which can develop into new cells, tissue, fat, bone, or cartilage.

As we grow older, red marrow is replaced by yellow marrow and adults will have more yellow rather than red bone marrow.

What is leukemia?
Leukemia is caused when the number of white blood cells in the bone marrow increases and crowds over the red blood cells and platelets. These new white blood cells cannot fight any infection as effectively as the original white blood cells. With the continuous growth of white blood cells, the production of red blood cells and platelets reduce. This means many parts of our body and vital organs do not get the required oxygen to perform their normal functions, while loss of platelets means the blood would not clot.

A person is diagnosed with leukemia when the body has more white blood cells than it needs.

Classification of leukemia
Leukemia is classified by the speed at which it spreads and which blood cell is affected. The different types of leukemia are classified based on the progress:

• Acute leukemia
  When blood cells do not mature and are unable to perform their functions, it is termed as acute leukemia, which spreads rapidly.
• Chronic leukemia
  When only some cells are immature and the rest of the blood cells are still able to perform their assigned functions, the body still works normally. In chronic leukemia, the leukemia could keep spreading but much slower compared to acute leukemia.

Types of leukemia

The different types of leukemia are:

• Acute lymphocytic leukemia – ALL
  This begins in the B- or T-lymphocytes, which is a type of white blood cell that builds our immunity. This is more common in younger children, and can affect the bone marrow throughout the body.
• Acute myeloid leukemia – AML
  It can affect any component of the blood – that is the red cells, white cells, or platelets. This spreads rapidly and is more common in adults. The cancer is not restricted to the bone marrow and can spread to other parts of the body too.
• Chronic lymphocytic leukemia – CLL
  This starts in the B-lymphocytes. One in every three types of leukemia is CLL and is more common in adults. Whereas one type of CLL spreads rapidly, in the other type, symptoms do not manifest even for a year after being affected.
• Chronic myeloid leukemia – CML
  This occurs when myeloid cells turn into immature cancer cells due to a genetic change. They grow slowly and then overpower the red cells and platelets.
• Hairy cell leukemia
  Hairy cell leukemia is a rare type of leukemia when too many B-lymphocytes are produced. It grows slowly and can go undetected for many years. Middle-aged men are more susceptible to hairy cell leukemia as compared to women.