79. Histology | Immune System: Thymus, Lymph Node, and Spleen

A patient presents with enlarged, tender lymph nodes in the axilla following a skin infection. Histology reveals numerous macrophages and dendritic cells filtering lymph as it travels from afferent to efferent vessels. Through which sequence does lymph normally flow inside a node?

A. Subcapsular sinus → trabecular sinus → medullary sinus → efferent vessel
B. Medullary sinus → subcapsular sinus → trabecular sinus → efferent vessel
C. Trabecular sinus → subcapsular sinus → medullary sinus → efferent vessel
D. Efferent vessel → subcapsular sinus → medullary sinus → thoracic duct
E. Trabecular sinus → medullary sinus → subcapsular sinus → efferent vessel

A

• A (✅) → Correct normal lymph flow:
  Afferent vessels → Subcapsular sinus → Trabecular sinuses → Medullary sinuses → Efferent vessel → Thoracic duct → Venous circulation.

• B–E → All have incorrect sequence; none follow physiologic direction of flow.

A 3-month-old infant presents with recurrent viral and fungal infections. Chest X-ray reveals an absence of the thymic shadow. Flow cytometry shows profound T-cell deficiency, but B-cell numbers are normal. Which developmental defect best explains this patient’s condition?

A. Failure of 3rd and 4th pharyngeal pouch formation
B. Failure of 1st and 2nd pharyngeal arch development
C. Defective MHC I expression on thymic epithelium
D. Absence of bone-marrow–derived progenitor migration
E. Loss of spleen capsule integrity

A

• A (✅) → Classic DiGeorge syndrome: thymic aplasia from failure of the 3rd and 4th pharyngeal pouches, causing absent thymus and parathyroids → ↓T-cell immunity with recurrent viral/fungal infections.

• B → 1st & 2nd arches → facial structures, not thymus.

• C → MHC I defect would impair CD8⁺ T-cell selection, but not cause thymic aplasia or absent shadow.

• D → Would reduce all lymphoid cell lines (both B & T), not selective T-cell deficiency.

• E → Affects spleen architecture, not thymic development.

During T-cell development, thymocytes that bind self-antigens with high affinity undergo apoptosis. This process is mediated by which specific thymic cell type?

A. Type II epithelioreticular cells
B. Type III epithelioreticular cells
C. Type V epithelioreticular cells
D. Type VI epithelioreticular cells
E. Type I epithelioreticular cells

C

• C (✅) → Type V cells in the medulla perform negative selection, eliminating T cells that strongly bind self-antigen on MHC II — essential for central tolerance.

• A/B → Type II/III mediate positive selection in cortex, ensuring MHC recognition.

• D → Type VI form Hassall’s corpuscles; secrete cytokines for maturation.

• E → Type I create the blood-thymus barrier at cortical boundary.

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