U01.02.002 Lymph node

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Learning Objectives

Master the structural and functional organization of the Lymph Node. Identify the specific histological zones—the Follicle, Paracortex, and Medulla—and correlate their structure with the circulation and activation of B and T cells for the USMLE Step 1.

1. General Lymph Node Structure

A lymph node is an encapsulated secondary ( $2^\circ$ ) lymphoid organ. It acts as a biological “checkpoint” for lymph before it returns to the systemic circulation.

Feature	Description
Vascularity	Many afferent vessels (entry); $1$ or more efferent vessels (exit).
Architecture	Encapsulated, with trabeculae (structural partitions) providing internal support.
Functions	Nonspecific filtration by macrophages, circulation of lymphocytes, and activation of the immune response.

2. The Cortex and Follicles (B-Cell Zone)

The outer cortex is primarily composed of follicles, which are the sites of B-cell localization and proliferation.

Follicle Type	Characteristics
$1^\circ$ Follicles	Dense and quiescent (inactive).
$2^\circ$ Follicles	Active; characterized by pale central germinal centers where B cells proliferate and undergo isotype switching.

3. The Paracortex (T-Cell Zone)

The paracortex is the region between the outer cortex (follicles) and the inner medulla. It is the primary site for T-cell interaction.

Anatomy/Clinical	Significance
High Endothelial Venules (HEV)	The specialized entry point where T and B cells exit the blood and enter the lymph node.
DiGeorge Syndrome	The paracortex is underdeveloped due to a lack of mature T cells.
Viral Infection (EBV)	Triggers an extreme cellular immune response, causing paracortical hyperplasia and lymphadenopathy.

4. The Medulla

The innermost part of the lymph node consists of cords and sinuses that facilitate filtration and exit.

Region	Contents
Medullary Cords	Closely packed lymphocytes and plasma cells.
Medullary Sinuses	Contain reticular cells and macrophages. These communicate directly with efferent lymphatics.

Activity

High-Yield Clinical Pearls:

Paracortex vs. Cortex: Remember that the paracortex is for T cells, and the follicles in the cortex are for B cells. If T cells are low (HIV, DiGeorge), look at the paracortex; if antibodies are low, look at the follicles.
Lymph Flow: Afferent lymphatic $\rightarrow$ Subcapsular sinus $\rightarrow$ Cortical sinus $\rightarrow$ Medullary sinus $\rightarrow$ Efferent lymphatic.
Viral Hyperplasia: Unlike bacterial infections, which often cause follicular hyperplasia (B cells), viral infections such as Mononucleosis often cause paracortical hyperplasia (T cells).