How Does the Skeletal System Work with the Immune System

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The skeletal system, comprising bones and the bone marrow, works with the immune system to support the body’s optimal functions.

The skeletal system helps to

  1. Produce immune cells
  2. Activate and signal immune cells
  3. Store immune cells for long-term future responses.
  4. Protects the immune organs.

We often assume bones to be just hard structures meant only for structural support. But our skeletal system plays a vital role in the body’s immune defense, too.

Ignoring this hidden connection directly impacts our ability to fight infections.

Many suffer from recurring illnesses due to weak immunity without realizing the fact that the problem might have begun in the bones themselves, especially in the bone marrow.

So, exploring the fascinating interaction between the skeletal system and the immune system helps us understand how the life-saving immune cells are formed and maintained in the body.

How the Skeletal System interacts with the Immune System

1. Production of immune cells by the Bone marrow:

The red bone marrow is the site of blood cell synthesis, where the

  • Red blood cells,
  • white blood cells, and
  • Platelets

are produced and released into the blood circulation.

bone marrow source of wbcs in immunity
White blood cells formed from red bone marrow.

Of these blood cells, white blood cells are key components of the body’s immunity. They perform diverse immune actions like

  1. Neutrophils- Engulf pathogenic microbial cells.
  2. Monocytes- Engulf the pathogen cells.
  3. Lymphocytes- Produce antibodies that attack pathogens and their antigens to neutralize.
  4. Eosinophils- These WBC cells attack pathogens like worms.
  5. Basophils- These WBCs are involved in inflammation as a part of the immune reaction.
  • These cells are generated from the pluripotent stem cells in the red bone marrow, where they originate, mature, and get released into the blood.
  • Any damage to bone marrow decreases immunity.

2. Activation and coordination of immune cells

  • The skeletal system activates the immune cells and also helps in their signaling.

a. Bone is a calcium reservoir supporting immunity

  • Bones store 99% of the body’s calcium.
  • This calcium from bones is released into the blood during deficiency.
  • It helps in the optimal body functions like immune function, besides nervous conduction and muscle contraction.

The immune system relies on calcium for

  • Activation, proliferation of immune cells
  • Cytokine production and phagocytosis
  • Differentiating self and non-self tissues to avoid autoimmunity or self-damage.
  • It is found that elevation of calcium is linked to immune system activation.

Any deficiency in the calcium levels results in inhibition of lymphocyte production that produces antibodies and neutrophil activation, which directly eat up pathogenic bacteria.

B and T cells production bone marrow schema

b. Bone as phosphate reservoir (PO4-) is essential for immunity

  • Phosphate is another molecule largely available from bones.
  • About 90% of phosphate is present in bones and is released to maintain phosphate levels in the blood.
  • This phosphate is involved in immune mechanisms like
  1. Immune cell migration and lymphocyte trafficking
  2. Immune cell communication and signalling through Sphingosine-1-Phosphate (
  3. Removal of apoptotic cells (efferocytosis)

3. Bone marrow – Storage site for B and T lymphocytes

  • Bone marrow acts as a storage site for immune cells, such as the antigen-specific memory B and T cells.
  • These immune cells are maintained for a long time in order to activate quickly to counter subsequent infections or pathogens in future exposures.
  • This contributes to long-term immunity and supports even the vaccination process.

4. Role in Protecting Immune-Related Organs

Thymus gland of immune system in rib cage protection
  • Besides supporting immune cells, the skeletal system also gives protection to vital organs related to the immune system.
  • The rib cage, made of bones, shields the thymus gland where T cells mature.
  • The bony skull encases the brain, which regulates the immune responses through the nervous and endocrine systems.

How the immune system works with the skeletal system.

1. Bone remodelling

Remodelling of bones is a process of removing old and damaged portions of the bone and replacing them with new bone tissue.

  • Osteoclasts are large cells with around 50 nuclei that are formed due to the fusion of many monocytes (a type of WBC cell).
  • These osteoclasts help in the breakdown of damaged bone tissue, releasing calcium and phosphate.

This activity is particularly in areas of bone with active growth, repair, and remodeling regions.

2. Bone repair

When there is a bone fracture or damage, the immune system helps in repair through a series of processes like

  • Inflammation,
  • Clearing of dead cells and debris, and
  • Fibrinous/cartilaginous tissue production and
  • Populating Osteoclasts.

This way immune system helps in bone repair.

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