STEPTRACKERIMMUNOLOGY + ANATOMY | CELLS & ORGANS | HIGH-YIELD REVIEW

Lymphoid Organs

Primary organs make lymphocytes; secondary organs are where they meet antigen. Know which zone holds B cells, which holds T cells, and what drains where.

FOCUS ON
Primary: bone marrow (B-cell maturation), thymus (T-cell maturation; cortex = immature, medulla = mature + Hassall corpuscles; from 3rd pharyngeal pouch). Lymph node: follicles in the cortex = B cells (germinal centers when active), paracortex = T cells (and high endothelial venules; enlarges in viral infection, underdeveloped in DiGeorge), medulla = cords of plasma cells and sinuses with macrophages. Spleen: white pulp (PALS = T cells, follicles = B cells, marginal zone) and red pulp (filters old RBCs); removes encapsulated bacteria → asplenia risk. MALT: Peyer patches (M cells, IgA). Drainage: lateral breast → axillary, testes → para-aortic, scrotum → superficial inguinal.
STRUCTURE
CONTENTS · FUNCTION
CLINICAL LINK
CLUE
Bone marrow
Hematopoietic stem cells; B-cell maturation; long-lived plasma cells
Aplastic anemia, leukemia, transplant source
Primary lymphoid organ
Thymus — cortex
Immature (double-positive CD4⁺CD8⁺) thymocytes; positive selection on cortical epithelium
Absent in DiGeorge (22q11) → T-cell deficiency
Densely packed, dark
Thymus — medulla
Mature single-positive T cells; negative selection (AIRE); Hassall corpuscles
Thymoma → myasthenia gravis, pure red cell aplasia
Pale, Hassall corpuscles
Lymph node — follicle
B cells; primary follicle (resting) → secondary follicle with germinal center (proliferating, class switching)
Follicular hyperplasia (reactive), follicular lymphoma
Outer cortex
Lymph node — paracortex
T cells, dendritic cells; high endothelial venules let lymphocytes enter from blood
Expands in viral infections (EBV); poorly developed in DiGeorge
Between follicles and medulla
Lymph node — medulla
Medullary cords (plasma cells, lymphocytes) and sinuses (macrophages, reticular cells)
Sinus histiocytosis in draining nodes
Exit via efferent lymphatic
Spleen — white pulp
Periarteriolar lymphoid sheath (T cells), follicles (B cells), marginal zone (B cells, macrophages that clear encapsulated bacteria)
Asplenia → pneumococcus, Hib, meningococcus
Around central arterioles
Spleen — red pulp
Cords of Billroth and sinusoids; macrophages remove old/abnormal RBCs
Hereditary spherocytosis, sickle cell sequestration
After splenectomy: Howell-Jolly bodies, target cells, thrombocytosis
MALT · Peyer patches
Unencapsulated lymphoid tissue; M cells sample gut antigen → IgA
Salmonella and Shigella enter via M cells; MALT lymphoma (H pylori)
Ileum, antimesenteric border
Tonsils
Lymphoid tissue in the pharynx (Waldeyer ring)
Absent in XLA
Not encapsulated on the pharyngeal side
Lymphatic drainage
Lymph flows afferent → node → efferent → thoracic duct (or right lymphatic duct) → venous angle
Lateral breast → axillary · upper limb → axillary · stomach → celiac · testes → para-aortic · scrotum, vulva, lower anus → superficial inguinal
Testis vs scrotum differ

Picture it — where B and T cells live

hilumafferent lymphmedullafollicle (B)paracortex (T)white dot = germinal center
Lymph nodeB cells sit in cortical follicles (germinal centers when active); T cells fill the paracortex; plasma cells and macrophages line the medulla near the hilum.
red pulpPALS (T)follicle (B)marginalzonecentralarteriolesplenic white pulp
Spleen white pulpT cells sheath the central arteriole (PALS), B cells form follicles, and the marginal zone traps blood-borne encapsulated bacteria.

Schematic diagrams, not to scale.

Mechanism — how lymphocytes find antigen

Dendritic cell picks up antigenIn tissue
→
Travels in afferent lymphCCR7
→
Paracortex: meets naive T cellsEntered via HEVs
→
T cells help B cells at follicle edgeGerminal center forms

Zones at a glance

ORGAN
B-CELL ZONE
T-CELL ZONE
Lymph node
Cortical follicles
Paracortex
Spleen
Follicles, marginal zone
PALS
Peyer patch
Follicle (dome)
Interfollicular
Thymus
—
Cortex (immature), medulla (mature)

How it's tested

COMMON CONFUSIONA node biopsy in XLA lacks follicles and germinal centers; in DiGeorge the paracortex is thin — match the missing zone to the missing cell.
THE TRAPTesticular cancer spreads first to para-aortic nodes (the testis descended from the abdomen), not to inguinal nodes like scrotal skin.
YOUR APPROACHName the cell type → its home zone → what enlarges or disappears in disease.

If you see… think…

Germinal centers in cortical follicles → activated B cells
Expanded paracortex in mono → T-cell proliferation
Absent thymic shadow on newborn X-ray → DiGeorge or SCID
Embryologic origin of the thymus → 3rd pharyngeal pouch (endoderm)
Hassall corpuscles → thymic medulla
Howell-Jolly bodies on smear → asplenia
Splenic area that clears encapsulated bacteria → marginal zone
Cells sampling gut antigen over Peyer patches → M cells
Testicular seminoma first spreads to → para-aortic nodes
Myasthenia gravis + anterior mediastinal mass → thymoma