Primary and secondary lymphoid organs !
- Topic
Immune cells are born and trained in the bone marrow and thymus, then meet invaders in lymph nodes, the spleen and the mucosal tissue.
Updated 2026-11-01
See it in 3D: Thymus · Spleen · Neck lymph nodes · Mesenteric lymph nodes · Palatine tonsils
What it is
Lymphoid organs are where lymphocytes are made, selected and put to work. The primary organs (bone marrow and thymus) produce lymphocytes and teach them to ignore the body's own tissues. The secondary organs (lymph nodes, spleen and mucosa-associated lymphoid tissue) are meeting points where naive lymphocytes find their antigen and start a response.
Key facts
- All lymphocytes start as hematopoietic stem cells in red bone marrow.
- Selection in primary organs keeps cells that can recognize antigen and removes most that react strongly against self.
- Mature naive lymphocytes circulate between blood and secondary organs until they meet their antigen.
- Germinal centers (inside B-cell follicles) are where B cells multiply and improve their antibodies.
The lymphoid organs at a glance
| Organ | Type | What happens there | Exam clue |
|---|---|---|---|
| Bone marrow | Primary | All blood cells are made; B cells mature | Marrow failure lowers every cell line |
| Thymus | Primary | T cells are selected and mature | Absent thymic shadow in DiGeorge or SCID |
| Lymph node | Secondary | Filters lymph; B and T cells meet antigen | Tender node: infection; hard, fixed node: think tumor |
| Spleen | Secondary | Filters blood; removes old red cells and coated bacteria | Asplenia: encapsulated bacteria |
| MALT (tonsils, Peyer patches) | Secondary | Samples antigens at mucosal surfaces; IgA | M cells over Peyer patches in the ileum |
Test yourself
Think of the answer, then tap the question to see it.
Which two organs are the primary lymphoid organs?
The bone marrow (where B cells mature) and the thymus (where T cells mature).
What does a lymph node filter, and what does the spleen filter?
Lymph nodes filter lymph; the spleen filters blood.
Where in a secondary lymphoid organ do B cells multiply and improve their antibodies?
In the germinal centers of the B-cell follicles.
What are three kinds of secondary lymphoid tissue?
Lymph nodes, the spleen and mucosa-associated lymphoid tissue (tonsils, Peyer patches, bronchial lymphoid tissue).
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Bone marrow and hematopoiesis of immune cells
- Thymus: where T cells learn
- Lymph node: cortex, paracortex, medulla
- Spleen: white pulp, red pulp, asplenia
- Mucosa-associated lymphoid tissue (tonsils, Peyer patches)
- B cell and plasma cell
- CD4⁺ helper T cell
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: 21.1 Anatomy of the Lymphatic and Immune Systems (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); 18.2 Production of the Formed Elements (OpenStax, Rice University (Anatomy and Physiology 2e), 2022).
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Representative model, for educational purposes only.