U01.02.002 Lymph node

Lymph nodes are secondary lymphoid organs involved in immune system regulation. They are encapsulated structures with multiple afferent lymphatic vessels and at least one efferent vessel, facilitating the filtration of lymph. Lymph nodes play a key role in immune surveillance, lymphocyte circulation, and the initiation of immune responses.

Structure of a Lymph Node

Lymph nodes are made up of distinct regions, each with specific functions related to immune defense.

1. Follicle

Location: Found in the outer cortex of the lymph node.
Function: It is the site of B-cell localization and proliferation.
- Primary (1°) follicles: Dense and inactive.
- Secondary (2°) follicles: Have pale central germinal centers, which indicate active proliferation and maturation of B-cells.

2. Medulla

Components:
- Medullary cords: Comprised of tightly packed lymphocytes and plasma cells.
- Medullary sinuses: These communicate with the efferent lymphatic vessels and are rich in reticular cells and macrophages, assisting in the filtration of lymph.

3. Paracortex

Location: The region between the follicles and the medulla.
Function:
- Contains T cells and is crucial for cellular immunity.
- High endothelial venules (HEV) in this region allow the entry of both B and T cells from the blood into the lymph node.
- The paracortex is not well-developed in patients with DiGeorge syndrome, who suffer from T-cell deficiency.
- This area undergoes hyperplasia during intense cellular immune responses, such as in infections caused by Epstein-Barr virus (EBV), leading to noticeable lymphadenopathy.

Function of Lymph Nodes

Lymph nodes perform three primary functions:

Nonspecific Filtration: By macrophages that trap and destroy foreign particles.
Circulation of Lymphocytes: B and T cells circulate through lymph nodes, ensuring immune surveillance.
Immune Response Activation: Lymph nodes are sites where immune responses are initiated, particularly when antigens are detected in the lymph.

Table: Components of Lymph Node

Region	Main Components	Primary Function
Follicle	B-cells (Primary and secondary follicles)	B-cell proliferation and maturation
Medulla	Medullary cords (lymphocytes, plasma cells)	Lymph filtration and immune defense
Paracortex	T-cells, High endothelial venules	T-cell activation and entry of lymphocytes

Table: Components of Lymph Node

Points to Remember

Lymph nodes filter lymph and play a crucial role in immune response.
Follicles are key for B-cell activity, while the paracortex is essential for T-cell function.
The medulla serves as a passage for lymphatic fluid and contains immune cells for nonspecific filtration.
Enlargement of the paracortex (paracortical hyperplasia) can be seen in viral infections like EBV.
Patients with DiGeorge syndrome lack a fully developed paracortex, resulting in immune deficiencies.

Bibliography

Abbas, A. K., & Lichtman, A. H. (2021). Basic Immunology: Functions and Disorders of the Immune System. Elsevier Health Sciences.
Janeway, C. A., et al. (2017). Immunobiology: The Immune System in Health and Disease. Garland Science.
Kumar, V., Abbas, A. K., & Aster, J. C. (2021). Robbins and Cotran Pathologic Basis of Disease. Elsevier.

USMLE Step 1 Review Series

Curriculum

U01.02.002 Lymph node

Structure of a Lymph Node

1. Follicle

2. Medulla

3. Paracortex

Function of Lymph Nodes

Table: Components of Lymph Node

Points to Remember

Bibliography

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