U01.02.002 Lymph node

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By the end of this session, students should be able to identify and describe the structural organization and immune functions of lymph nodes and correlate these with clinical conditions such as DiGeorge syndrome and viral lymphadenopathy.

The lymph node is a secondary (2°) lymphoid organ that filters lymph and coordinates immune responses. It is encapsulated with trabeculae extending inward.

Key Functions

Function	Description
Filtration	Nonspecific filtration of lymph by macrophages to trap pathogens.
Immune Surveillance	Circulation and interaction of B and T lymphocytes.
Activation	Initiation of adaptive immune responses upon antigen exposure.

Structural Components of a Lymph Node

Region	Location	Major Cells	Key Features	Clinical Correlate
Follicles	Outer cortex	B cells	Site of B-cell localization and proliferation. Primary follicles – dense and quiescent. Secondary follicles – have pale germinal centers (active).	Hyperplasia in humoral immune responses (e.g., bacterial infections).
Paracortex	Between follicles and medulla	T cells	Contains high endothelial venules (HEVs) for lymphocyte entry from blood.	Underdeveloped in DiGeorge syndrome due to T-cell deficiency. Enlarges in viral infections (e.g., EBV).
Medulla	Inner region	Lymphocytes, plasma cells, macrophages	Contains medullary cords (cells) and medullary sinuses (spaces) communicating with efferent lymphatics.	Metastatic cancer cells or infectious agents can accumulate here.

Region

Location

Major Cells

Key Features

Clinical Correlate

Follicles

Outer cortex

B cells

Site of B-cell localization and proliferation.

Primary follicles – dense and quiescent.
Secondary follicles – have pale germinal centers (active).

Hyperplasia in humoral immune responses (e.g., bacterial infections).

Paracortex

Between follicles and medulla

T cells

Contains high endothelial venules (HEVs) for lymphocyte entry from blood.

Underdeveloped in DiGeorge syndrome due to T-cell deficiency. Enlarges in viral infections (e.g., EBV).

Medulla

Inner region

Lymphocytes, plasma cells, macrophages

Contains medullary cords (cells) and medullary sinuses (spaces) communicating with efferent lymphatics.

Metastatic cancer cells or infectious agents can accumulate here.

Key Points

Afferent lymphatics bring lymph into the node; efferent lymphatics carry it away (at the hilum).
Encapsulation distinguishes lymph nodes from non-encapsulated lymphoid tissues (e.g., tonsils, MALT).
Cortex = B cells, Paracortex = T cells, Medulla = Plasma cells + Macrophages.
Paracortical hyperplasia → sign of cellular immune activation, common in viral infections.

Clinical Correlation Highlights

Condition	Pathophysiology	Histologic Feature	Clinical Presentation
DiGeorge Syndrome	22q11 deletion → thymic hypoplasia → ↓ T cells	Underdeveloped paracortex	Recurrent viral/fungal infections
EBV Infection	B-cell infection → T-cell activation	Paracortical hyperplasia	Tender cervical lymphadenopathy
Metastatic Cancer	Tumor cell migration via lymphatics	Tumor deposits in medullary sinuses	Firm, non-tender lymph node

Summary Table: Lymph Node Organization

Structure	Cell Type	Function	Clinical Note
Follicle	B cells	Antibody production	Enlarged in bacterial infections
Paracortex	T cells	Cell-mediated immunity	Absent in DiGeorge syndrome
Medulla	Plasma cells, macrophages	Filtration, antibody secretion	Site of metastasis
Capsule & Trabeculae	Connective tissue	Support and compartmentalization	—