Lymph Node and Splenic Architecture

Lymph nodes and the spleen are the major secondary lymphoid organs, where mature lymphocytes are positioned to encounter antigen. Both are organized into compartments, B-cell follicles and T-cell zones, but they sample different fluids: lymph nodes filter lymph draining from tissues, while the spleen filters blood and also removes aged red cells.

Definition

Lymph node and splenic architecture refers to the compartmentalized microscopic organization of these secondary lymphoid organs into B-cell follicles, T-cell zones, and specialized filtering regions that position immune cells to detect and respond to antigen.

Scope

This topic covers the histological compartments of the lymph node and the spleen and how their architecture brings antigen and lymphocytes together. It is a structural overview of secondary lymphoid organs; it does not address lymphadenopathy, splenomegaly, or their clinical management.

Core questions

• How is a lymph node organized into cortex, paracortex, and medulla?
• How is the spleen divided into white pulp and red pulp, and what does each do?
• How are B-cell and T-cell compartments established and maintained?
• How do these organs sample lymph and blood for antigen?

Key concepts

• Secondary lymphoid organ
• B-cell follicles and germinal centers
• T-cell (paracortical) zones
• Splenic white pulp and red pulp
• Stromal cells and conduit/reticular network
• Chemokine-guided compartmentalization

Mechanisms

The lymph node has an outer cortex rich in B-cell follicles, a deeper paracortex populated by T cells, and a medulla through which lymph drains toward efferent vessels; afferent lymph delivers antigen and antigen-presenting cells from the tissues. The spleen is built around the white pulp, lymphoid sheaths and follicles surrounding arterioles, and the red pulp, where blood percolates through cords and sinuses and aged erythrocytes are removed. In both organs a stromal cell network organizes the tissue and, together with chemokine gradients, segregates B and T lymphocytes into their distinct compartments so that the right cells meet antigen.

Clinical relevance

Knowing the normal compartments of the lymph node and spleen underlies the histological reading of these organs in pathology and the interpretation of imaging and biopsy. As a reference topic it describes normal architecture; it is not a basis for diagnosing or managing enlarged nodes, an enlarged spleen, or any specific disease.

History

The gross compartments of nodes and spleen were described by classical histology, but the molecular basis of their organization emerged later, when stromal cell networks and chemokine gradients were shown to position lymphocytes into separate B-cell and T-cell zones. This reframed secondary lymphoid organs as actively structured environments rather than passive filters.

Key figures

• Reina Mebius
• Georg Kraal
• Ronald Germain
• Andrew Luster

Seminal works

• mebius-kraal-2005
• mueller-germain-2009

Frequently asked questions

What is the difference between the white pulp and red pulp of the spleen?

The white pulp is the lymphoid tissue around splenic arterioles where immune responses occur, while the red pulp is the blood-filled tissue where aged or damaged red cells are filtered and removed.

Why are B cells and T cells found in separate regions of a lymph node?

Stromal cells and the chemokine gradients they produce guide B and T lymphocytes into distinct follicles and paracortical zones, organizing the encounter with antigen.