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Immunology · Lymphoid organs

Bone marrow and hematopoiesis of immune cells !

  • Organ

The red bone marrow is the factory of every blood cell, including all white cells, and the school where B cells mature.

Updated 2026-11-01

See it in 3D: Sternum · Hip bone · Femur

What it is

Red bone marrow, inside spongy bone, holds the hematopoietic stem cells that make red cells, platelets and all white cells. It is a primary lymphoid organ: B cells complete their development there, while T-cell precursors leave for the thymus. Many long-lived plasma cells return to the marrow to keep making antibody, which is why a plasma-cell cancer (multiple myeloma) grows in bone.

Structure

  • Red marrow: a meshwork of reticular tissue and wide capillaries (sinusoids) packed with developing blood cells and fat cells.
  • Yellow marrow: mostly fat; it replaces red marrow in the shafts of long bones with age.
  • Precursors are named with the suffix -blast (myeloblast, lymphoblast) before they mature.

Function

  • Makes neutrophils, eosinophils, basophils and monocytes from the myeloid line.
  • Makes B cells, NK cells and T-cell precursors from the lymphoid line.
  • Removes most B cells that bind self-antigen strongly (central tolerance).
  • Houses long-lived plasma cells that keep antibody levels up for years.

Step by step

  1. A hematopoietic stem cell divides: one daughter stays a stem cell, the other commits to a line.
  2. Growth factors (erythropoietin, thrombopoietin, colony-stimulating factors, interleukins) steer the committed cell.
  3. Myeloid progenitors become granulocytes, monocytes, red cells and platelet-forming megakaryocytes.
  4. Lymphoid progenitors become B cells and NK cells in the marrow; T-cell precursors travel to the thymus.
  5. Mature cells cross the sinusoid walls and enter the blood.

Clinical notes

  • Multiple myeloma: plasma-cell cancer of the marrow; common in older adults; anemia, bone lesions and infections.
  • Leukemias start from a marrow precursor; the blood shows abnormal or immature cells and low normal cells.
  • Bone marrow transplant replaces a diseased marrow with donor stem cells and needs a matching donor.
  • Extramedullary hematopoiesis: when marrow fails, the liver and spleen may make blood cells again and enlarge.

In the lab

  • Complete blood count with differential: first look at marrow output.
  • Bone marrow aspirate and biopsy, usually from the posterior iliac crest.
  • Myeloma work-up: serum and urine protein electrophoresis, immunofixation and free light chains.

Test yourself

Think of the answer, then tap the question to see it.

Which two lineages arise from the hematopoietic stem cell?

The myeloid line (red cells, platelets, granulocytes, monocytes) and the lymphoid line (B, T and NK cells).

Where do B cells and T cells complete their maturation?

B cells in the bone marrow; T cells in the thymus.

Which adult bones hold most of the red marrow?

The axial skeleton (skull, vertebrae, sternum, ribs, pelvis) and the upper ends of the femur and humerus.

Why does multiple myeloma cause anemia and infections?

Malignant plasma cells crowd the marrow, so fewer normal red cells and white cells are made, and normal antibodies fall.

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← All of immunology

Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.

Sources: 18.2 Production of the Formed Elements (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); 18.4 Leukocytes and Platelets (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); Multiple myeloma (MedlinePlus, U.S. National Library of Medicine, 2024).

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