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Different types of leukemia start in different types of blood cells. It can help to know some basics about the bone marrow and blood cells.

Bone marrow

Blood cells are made in your bone marrow.

Bone marrow is the soft inner part of some bones like the skull, shoulder blades, ribs, pelvis, and backbones. Bone marrow is made up of:

• A small number of blood stem cells
• More mature blood-forming cells
• Fat cells
• Supporting tissues that help cells and blood vessels grow

Inside the bone marrow, blood stem cells divide and mature to make new blood cells. During this process, the cells become:

• Lymphocytes (a kind of white blood cell)
• Other blood-forming myeloid cells. These myeloid cells can develop red blood cells, white blood cells (other than lymphocytes), or platelets.

Chronic myeloid leukemia (CML) develops in early forms of myeloid cells.

Types of blood cells

The different types of blood cells each have different jobs in the body.

Red blood cells

Red blood cells carry oxygen from the lungs to all other tissues in the body and take carbon dioxide back to the lungs to be removed.

Having too few red blood cells in the body (anemia) can make you feel tired, weak, and short of breath because your body tissues aren’t getting enough oxygen.

Platelets

Platelets are pieces of cells that break off from a large bone marrow cell called the megakaryocyte. Platelets are important in plugging up holes in blood vessels caused by cuts or bruises to help stop bleeding.

Having too few platelets (thrombocytopenia) may cause you to bleed or bruise easily.

White blood cells

White blood cells help the body fight infections.

Having too few white blood cells lowers your immune system and can make you more likely to get an infection.

Types of white blood cells

There are several types of white blood cells.

Lymphocytes

Lymphocytes are mature, infection-fighting cells that develop from lymphoblasts, a type of blood stem cell in the bone marrow. Lymphocytes are the main cells that make up lymphoid tissue, a major part of the immune system.

Lymphoid tissue is found in lymph nodes, the thymus, the spleen, the tonsils, the adenoids, and is scattered throughout the digestive and respiratory systems and the bone marrow.

The 2 major types of lymphocytes are:

• B lymphocytes (B cells)
• T lymphocytes (T cells)

Lymphocytes help protect your body from germs. Some types of lymphocytes help regulate the immune system.

Granulocytes

Granulocytes are mature, infection-fighting cells that develop from myeloblasts, a type of blood-forming cell in the bone marrow.

Granulocytes have granules that show up as spots under the microscope. These granules contain enzymes and other substances that can destroy germs, such as bacteria.

There are 3 types of granulocytes:

• Neutrophils
• Basophils
• Eosinophils

These are distinguished under a microscope by the size and color of their granules.

Neutrophils are the most common type of granulocyte in the blood. They have a key role in fighting bacterial infections.

Monocytes

Monocytes develop from blood-forming monoblasts in the bone marrow and are related to granulocytes. After circulating in the bloodstream for about a day, monocytes enter body tissues to become macrophages.

Macrophages can destroy some germs by surrounding and digesting them. They also help lymphocytes recognize germs and start fighting them.