Childhood leukemia

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Anatomy and physiology of the blood

Blood is made up of liquid (called plasma) and solid cells. Blood cells are made in the bone marrow. Bone marrow is the soft, spongy substance in the centre of bones.


In adults, the most active bone marrow is found in the pelvic bone, shoulder bones, back bones (vertebra), ribs, breast bone and skull. Immature blood cells in the bone marrow are called stem cells. Stem cells can also be found in smaller amounts in the bloodstream. These are called peripheral blood stem cells.

All of our blood cells develop from stem cells. The process of blood cell development is called hematopoiesis. In the earliest stage of blood cell development, stem cells begin to develop either along the lymphoid cell line or the myeloid cell line. In both cell lines, the stem cells become blasts, which are still immature cells.

Lymphoid cell line

Lymphoid stem cells develop into lymphocytes (also called lymphoblasts). Lymphocytes are another type of white blood cell. They make antibodies to help fight infection.

Myeloid cell line

Myeloid stem cells develop into red blood cells, platelets or white blood cells (also called leukocytes). Red blood cells carry oxygen to all tissues of the body.Platelets form clots in damaged blood vessels to prevent bleeding.

Myeloid stem cells can develop into 2 different types of white blood cells, called granulocytes and monocytes. These white blood cells destroy bacteria and help to fight infection.

Diagram of blood cell development


The main function of blood is to carry nutrients to and remove waste from cells, carry hormones to tissues and protect the body from harmful micro-organisms. Each type of blood cell has a specific job.

  • Red blood cells carry oxygen from the lungs to the rest of the body. They also carry carbon dioxide from the body to the lungs.
  • Platelets form blood clots in damaged vessels to stop bleeding.
  • White blood cells help prevent and fight infection by destroying bacteria, viruses and other foreign cells or substances.

Types of white blood cells

There are many different types of white blood cells. Each type has a different job.


Lymphocytes make antibodies to fight infection. They are found in the lymph nodes, thymus, spleen, tonsils, adenoids and the bone marrow. They are also in lymphatic tissue in other parts of the body such as the stomach and lining of the intestines.

The 3 main types of lymphocytes are:

  • B cells make antibodies to fight bacteria, viruses and fungi.
  • T cells activate the B cells to make antibodies.
  • Natural killer (NK) cells attack any foreign cells.


Granulocytes destroy bacteria to fight infection. The 3 main types of granulocytes are:

  • Neutrophils are the main cells that fight infection. They surround and eat, or ingest, foreign cells.
  • Eosinophils help control inflammation and allergic reactions. They attack and destroy certain parasitic organisms.
  • Basophils release chemicals to fight some types of infection and during allergic reactions.


Monocytes help fight infection by eating substances like bacteria and fungi.


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