1. Produce immune cells
Produces many circulating immune cells and the precursors of B, T and NK cells. T-cell development continues in the thymus.
IMMUNE TISSUES
Bone
Marrow
The Source of the Immune System.
Bone marrow is the primary site of blood cell production (hematopoiesis). It generates blood cells and many immune-cell precursors, including precursors that migrate to the thymus to become mature T cells. Some tissue-resident macrophages are established before birth and maintained by local self-renewal. Throughout life, bone marrow adapts to the body’s needs and plays a critical role in immune defense, tissue repair, and long-term immune health.
Back to all immune tissues →What Is the Bone Marrow?
The bone marrow is a soft, sponge-like tissue located in the cavities of most bones, particularly the spine, pelvis, sternum, ribs, and long bones (femur, humerus). It harbors hematopoietic stem and progenitor cells (HSPCs) that give rise to all blood cells, including the cells of the immune system.
Bone marrow provides a specialized microenvironment, or “niche”, composed of stromal cells, endothelial cells, extracellular matrix and signaling molecules, which regulate stem cell maintenance, differentiation and egress of mature cells into the bloodstream.
Its activity is tightly regulated and adapts to physiological demands, infection, inflammation, and aging.
Bone Marrow at a glance
Type of tissuePrimary hematopoietic tissue
Main roleProduce all blood cells, including immune cells
Where it isInside most bones (e.g., pelvis, sternum, ribs, spine, femur)
LifespanActive throughout life (adapts with age)
Key Functions of the Bone Marrow
The bone marrow performs multiple essential functions that sustain immunity and overall health.
2. Produce red blood cells and platelets
Also produces erythrocytes (red blood cells) and platelets, essential for oxygen transport and clotting.
3. Maintain immune diversity
Supports B-cell receptor rearrangement and selection, and supplies progenitors for T-cell development and receptor selection in the thymus.
4. Adapt to the body’s needs
Increases or shifts production in response to infection, inflammation, injury or other physiological demands.
Hematopoiesis: From Stem Cells to Immune Cells
Adult blood-cell production is sustained by hematopoietic stem cells in bone marrow. This simplified lineage diagram does not show embryonic origins or local self-renewal of tissue-resident immune populations.
Hematopoietic stem cell (HSC)
Myeloid progenitor
Lymphoid progenitor
Erythrocytes(red blood cells)
Platelets
MonocytesMacrophages
Dendritic cells
Dendritic cells
Neutrophils
Eosinophils
Basophils
B cells→ Plasma cells
T cells(CD4+, CD8+, Treg, γδ T cells)
Natural killer(NK) cells
Hematopoietic stem cell (HSC)
Myeloid progenitor
Erythrocytes
Red blood cells
Platelets
Monocytes
Can develop into macrophages and some dendritic cells
Neutrophils
Eosinophils
Basophils
Lymphoid progenitor
B cells
Can become plasma cells
T cells
CD4+, CD8+, Treg and γδ T cells
Natural killer cells
NK cells
T-cell precursors leave the bone marrow and mature in the thymus. This is a simplified lineage model; tissue-resident populations and dendritic-cell development have additional pathways.
Bone Marrow and Aging
A resilient immune system today
for healthier tomorrows.
for healthier tomorrows.
With age, several changes occur in the bone marrow:
- Decline in hematopoietic stem cell function and diversity.
- Skewing toward myeloid cell production (inflammaging).
- Reduced production of naïve B and T cells.
- Accumulation of DNA damage and altered niche function.
- These changes contribute to immunosenescence and increased susceptibility to infections, cancer and inflammatory diseases.
Bone Marrow in Health and Disease
Immune defenseEnsures continuous production of immune cells to fight infections.
Bone marrow failureReduced cell production leads to anemia, immunodeficiency and bleeding disorders (e.g., aplastic anemia, chemotherapy).
Blood cancersLeukemia starts in blood-forming tissue; lymphoma in lymphocytes. Myeloma affects plasma cells, usually in bone marrow.
TransplantationHematopoietic stem cell transplantation can reconstitute the immune system in various disorders.
Effect of the microenvironmentThe bone marrow niche regulates stem cell function and immune cell production.
The good news
Healthy lifestyle choices — including regular physical activity, a balanced diet, adequate sleep and stress management — can help support bone marrow function and healthy immune aging.
Learn more about aging & immunity →Selected sources and references
- Janeway et al., 2001 — The components of the immune system
- Fröbel et al., 2021 — The hematopoietic bone marrow niche ecosystem
- Inflamm-aging of hematopoiesis, hematopoietic stem cells, and the bone marrow microenvironment, 2016
- Origins, biology and diseases of tissue macrophages
Artwork illustrates the main mechanisms schematically; molecular sizes, colors and distances are not literal.