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Organs of the Immune System

Organa systematis immunis

For medical students2 min readUpdated 2026-10-10

The organs of the immune system are specialized anatomical structures that ensure the formation, maturation, and functioning of immune cells. They are divided into primary organs, where basic cell differentiation occurs, and secondary organs, where lymphocytes encounter foreign antigens and mount an immune response.

Bone marrowThe birthplace of all immune cells (hematopoiesis).
ThymusThe site of antigen-independent maturation of T lymphocytes.
Lymph nodesBiological filters where T- and B-cell responses are initiated.
SpleenThe main secondary lymphoid organ for peripheral immunity (humoral response).

Classification

Anatomo-functionally, the human immune system is divided into two major groups of organs. Their key distinction lies in whether the processes depend on contact with foreign agents (antigens).

  1. Primary (Central) organs: Bone marrow and thymus. This is where immune cells are born and undergo primary, antigen-independent maturation.
  2. Secondary (Peripheral) organs: Lymph nodes, spleen, Peyer's patches, as well as lymphoid tissues associated with the skin (SALT) and mucous membranes (MALT). Naive (untrained) lymphocytes migrate here. These organs host antigen-dependent processes: final maturation, antigen presentation, and execution of the immune response.

Primary Organs

Bone marrow consists of stroma and hematopoietic tissue. It is a reservoir of undifferentiated stem cells where hematopoiesis (myelopoiesis and lymphopoiesis) takes place. T- and B-lymphocytes, as well as natural killer (NK) cells, are generated here. B-lymphocytes leave for secondary organs and can return after encountering an antigen as plasma cells to synthesize antibodies.

The thymus is located posterior to the manubrium of the sternum. It is a bilobed organ responsible for the maturation and selection of T-lymphocytes. A thymic lobule consists of a cortex and a medulla, with the blood-thymus barrier located in the cortex. With age (after about 30 years), the organ undergoes involution, with tissue being replaced by adipose and connective tissue.

Secondary Organs and Tissues

Lymph nodes are oval structures located along lymphatic and blood vessels. They consist of a cortex (B-zone), paracortex (T-zone), and medulla. Upon contact with an antigen in the primary follicle, B-lymphocytes are activated, forming a germinal center (secondary follicle) where cells proliferate, differentiate into plasma cells, and undergo immunoglobulin class switching.

Peyer's patches are aggregated lymphoid follicles in the intestinal mucosa. Their epithelium contains M cells (microfold cells), which capture antigens from the lumen and deliver them to macrophages and dendritic cells. This is critical for initiating an appropriate mucosal immune response.

The spleen is a parenchymal organ in the abdominal cavity. In the fetus, it is responsible for hematopoiesis, but after birth, it becomes a crucial component of peripheral immunity (mainly humoral) and acts as a blood reservoir. Its white pulp features a T-zone (periarteriolar lymphatic sheath, PALS) and a B-zone (follicles).

Frequently asked questions

Which cells are produced in the bone marrow during hematopoiesis?

Myeloid and lymphoid lineage cells are produced in the bone marrow during hematopoiesis.

  • Myeloid lineage cells: Monocytes, macrophages, neutrophils, eosinophils, basophils, mast cells, dendritic cells, erythrocytes, and megakaryocytes.
  • Lymphoid lineage cells: T-lymphocyte subpopulations (CD8+, CD4+), B-lymphocytes (B1, B2), and NK cells (natural killers).
What structures make up the blood-thymus barrier?

The blood-thymus barrier is formed by blood vessels and thymic cortical stromal cells. It includes:

  • Blood capillaries and arterioles: Isolate maturing thymocytes from blood-borne antigens.
  • Supporting reticuloepithelial cells: Flat cells surrounding the vessels that create the structural framework of the cortex.
What functions does the adult spleen perform?

In adults, the spleen (lien, splen) performs several vital functions as a major secondary lymphoid organ:

  • Immune function: Adaptive response, antigen-dependent lymphocyte proliferation, antibody production, and antigen phagocytosis.
  • Reservoir function: Storage of blood (up to 300 mL), erythrocytes, granulocytes, and platelets (up to 30% of the pool).
  • Filtration function: Removal of senescent erythrocytes, platelets, and microorganisms.
  • Hematopoietic function: Site of lymphopoiesis.
  • Metabolic function: Iron recycling from hemoglobin of destroyed erythrocytes.
What anatomical structures comprise MALT?

Mucสาosa-associated lymphoid tissue (MALT) includes structures of the gastrointestinal, respiratory, and urogenital tracts.

  • Waldeyer's ring (pharyngeal lymphoid ring): Includes six tonsils (lingual, pharyngeal, two palatine, and two tubular).
  • Gut-associated lymphoid tissue (GALT): Solitary follicles and Peyer's patches in the small intestine wall, as well as aggregated lymphoid nodules in the appendix.
  • Bronchus-associated lymphoid tissue (BALT): Lymphoid nodules in the walls of the airways.

MALT also includes diffuse lymphoid tissue represented by scattered lymphocytes outside follicles.

What is the fundamental difference between primary and secondary immune organs?

In primary organs, cell differentiation occurs independently of antigens. In secondary organs, final maturation and the initiation of an immune response occur only after encountering a foreign agent.

Where do T-lymphocytes and B-lymphocytes mature?

T-lymphocytes mature in the thymus (hence 'T'). B-lymphocytes are formed and undergo primary development in the bone marrow (hence 'B' for bursa-equivalent in birds, but bone marrow in mammals).

What is a germinal center?

It is a site of intense B-lymphocyte proliferation and differentiation within a lymph node or Peyer's patch follicle that forms in response to an antigen.

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