Bone marrow and the immune system

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Parents can learn about bone marrow and the immune system, as they prepare for their child's blood and marrow transplant (BMT)/cellular therapy (CT).

Key points

  • Bone marrow is the spongy tissue inside bones.
  • Bone marrow produces red blood cells, platelets and white blood cells.
  • Lymphocytes, a type of white blood cell, are produced in the bone marrow and play an important part in the body's immune system.

Bone marrow is the spongy tissue inside our bones. All bones in newborn babies have active marrow, which means they are producing new marrow cells. By the time your child reaches young adulthood, the marrow inside the bones of the hands, feet, arms and legs stops producing new marrow cells. In adults, active marrow is found inside the spine, hip and shoulder bones, ribs, breastbone and skull. However, bone marrow found in the spine and hip has the richest source of bone marrow cells.

Bone marrow Cross section of bone identifying compact bone, spongy bone and bone marrow (contained in spongy bone)
Bone marrow is contained within bones. All blood cells are produced in the bone marrow.

What does the marrow do?

Our bone marrow produces blood cells, called red blood cells, platelets and white blood cells.

Inside the marrow, blood cells start off as young, immature cells called stem cells. Once they mature and go into circulation in the blood stream, blood cells do not live for a long time. This is why our marrow continuously produces all three types of blood cells to keep us healthy.

Red blood cells

These cells are red because they are filled with a protein called haemoglobin. Oxygen and carbon dioxide attach to the iron that is in haemoglobin, allowing the red blood cell to transport oxygen to the body. The red blood cells also get rid of carbon dioxide, which leaves your body through the lungs when you breathe out.


Platelets are blood cells that help the blood clot (stick together) to help stop bleeding at sites on the body that have been cut or injured. Platelets form the scab that is formed over a small cut.

White blood cells

White blood cells help the body fight against infection. There are many different types of white blood cells, which include:

  • Lymphocytes, neutrophils, and monocytes. These white blood cells fight against invading bacteria, viruses or fungi to help destroy infection. Each of these cells differs in appearance.
  • Eosinophils and basophils. These white blood cells respond to allergens that can invade our bodies.
The components of blood Eosinophil, red blood cell, lymphocyte, monocyte, plasma, platelet, neutrophil and basophil in blood
Blood is made up of red blood cells, platelets and different kinds of white blood cells. These cells are all suspended in liquid plasma.

Immune system

Our immune system protects the body from disease. It kills unwanted micro-organisms, such as bacteria and viruses, that may invade our bodies.

How does our immune system fight against infection?

Small glands called lymph nodes are scattered throughout the body. Lymphocytes, which are made inside our marrow, enter the lymph nodes.

The lymphocytes travel between each node through channels called lymphatics. The lymphatic channels meet at large ducts that empty into a blood vessel. Lymphocytes enter the blood through these ducts to help fight infection.

There are three major types of lymphocytes, which play an important part of the immune system.

B-lymphocytes (B-cells)

These cells originate in the bone marrow. They make proteins called antibodies, which attach onto the surface of infection-causing microbes. Generally, these are Y or T shaped. Each type of antibody reacts to different microbes by sticking to molecules, called antigens, which sit on the surface of the microbe. It is this antibody-antigen binding that triggers B-cells to grow and produce more antibodies, which fight infection.

T-lymphocytes (T-cells)

These cells mature in the thymus, which is a small organ in the upper chest, just behind the sternum (breastbone). T-cells help B-cells make antibodies against invading bacteria, viruses or other microbes. Unlike B-cells, T-cells engulf and destroy pathogens directly, after binding to the antigen on the surface of the microbe.

Natural killer (NK) cells

These are a type of lymphocyte that directly attacks cells that have been infected by a virus.

Last updated: June 20th 2023