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- Lymph node has cortex (follicles), paracortex (T-cell zone), medulla (cords/sinuses) with subcapsular and medullary sinuses.
- Spleen divides into white pulp (PALS + follicles) and red pulp (cords of Billroth with sinusoids).
- Thymus has cortex (immature T cells, Hassall corpuscles in medulla); involutes with age, gaining fat.
- Bone marrow shows trilineage hematopoiesis in intertrabecular spaces with sinusoidal vasculature.
- Capsule with subcapsular sinus; afferent lymphatics enter here.
- Secondary follicles with germinal centers (BCL6/CD10+) and mantle zones; paracortex T-cell rich (CD3+, high endothelial venules).
- Medullary cords plasma cells/macrophages; sinuses lined by CD68+/CD169+ histiocytes.
- White pulp: central arteriole with periarteriolar lymphoid sheath (T cells) and follicles (B cells); marginal zone around follicles.
- Red pulp: splenic cords (macrophages, plasma cells) and sinusoids with stave cells (CD8+ in some species, not human).
- No true afferent lymphatics; capsule has smooth muscle and elastic fibers.
- Lobulated; cortex basophilic with immature TdT+ thymocytes; medulla paler with Hassall corpuscles (keratinizing whorls).
- Epithelial cells CK19+; maturation from cortex to medulla; blood–thymus barrier in cortex.
- Bony trabeculae with hematopoietic cells between sinusoids; fat proportion increases with age/site.
- Myeloid:erythroid ratio roughly 2–4:1 in adults; megakaryocytes large, lobulated nuclei.
- Reticulin fibers delicate; sinusoidal endothelium CD34+, adventitial reticular cells.
- B cells: PAX5, CD20, CD79a; follicular dendritic cells CD21/CD23 in germinal centers.
- T cells: CD3; paracortical or PALS dominant.
- Macrophages: CD68/CD163; tingible body macrophages in germinal centers.
- Thymic epithelium: CK19, p63 (subset), keratin AE1/AE3; immature T cells TdT+.
- Marrow stroma: CD34 on endothelium; reticulin/silver stain for network assessment.
