SN2 SHORT NOTE 5M ★ High-yield

Describe the histology of the lymph node, spleen and thymus.

Anatomy · 1st Professional — Pre-Clinical

Lymphoid tissue is reticular connective tissue infiltrated with lymphocytes, concerned with immune defence.

Class	Organs	Function
Primary (central)	Bone marrow and thymus	Lymphocytes are produced and mature here; independent of antigen
Secondary (peripheral)	Lymph nodes, spleen, tonsils, Peyer patches, malt	Lymphocytes respond to antigen here

Region	Contents	Cells
Capsule and trabeculae	Dense connective tissue	—
Cortex	Lymphoid follicles with germinal centres	B lymphocytes
Paracortex	Diffuse lymphoid tissue; high endothelial venules	T lymphocytes
Medulla	Medullary cords and sinuses	Plasma cells and macrophages

Afferent lymphatics are many and pierce the convex surface; the efferent is single and leaves at the hilum
Germinal centres enlarge in antibody responses; the paracortex expands in viral infection and in cell-mediated responses — the distinction is visible on histology

Component	Structure	Function
White pulp	Periarteriolar lymphoid sheath (PALS) — T cells; lymphoid follicles — B cells	Immune function
Red pulp	Splenic cords (of Billroth) and venous sinusoids	Filtration; destruction of aged red cells
Marginal zone	Between the two	First contact with blood-borne antigen

The spleen has NO afferent lymphatics — it filters blood, not lymph, which distinguishes it from a lymph node at once
The sinusoids have a discontinuous endothelium with slits; red cells must deform to squeeze through, so rigid and aged cells are trapped and destroyed

Cortex — densely packed immature T lymphocytes (thymocytes); dark on low power
Medulla — fewer lymphocytes, and the hassall corpuscles of concentrically arranged epithelial cells — the diagnostic feature
Epithelial reticular cells form the framework and the blood–thymus barrier, and mediate positive and negative selection
It is at its largest relative to the body at birth, reaches its greatest absolute size at puberty, and then involutes and is replaced by fat
No lymphoid follicles and no germinal centres — another distinguishing feature

Lymphadenopathy — reactive nodes are soft, tender and mobile; malignant nodes hard, non-tender and fixed; tuberculous nodes matted with possible caseation
Splenectomy removes the filter for encapsulated organisms and carries a lifelong risk of overwhelming post-splenectomy infection; vaccination and prophylaxis are essential
DiGeorge syndrome — failure of the third and fourth pharyngeal pouches; absent thymus and parathyroids, giving T cell deficiency and hypocalcaemic tetany
Myasthenia gravis is associated with thymic hyperplasia or thymoma, and thymectomy often improves it
Lymphoma effaces the normal architecture of the node — loss of the follicular pattern on biopsy is the key diagnostic observation

All questions in General Histology