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Structure of the Lymph Node

Q: What types of antigen-presenting cells are present in lymphoid follicles (nodules)?

Lymphoid follicles contain the following cell populations: Macrophages — located within the germinal center and also found in the mantle zone (corona) of the follicle.Dendritic cells — part of the cellular population of the germinal center.Follicular dendritic cells — essential for the formation of follicular dendritic networks and lymphoid follicles, requiring lymphotoxin signaling.

Nodus lymphaticus

For medical students2 min readUpdated 2026-10-10

Lymph nodes are peripheral organs of the immune system that serve as the second barrier against antigens (after mucous membranes). Their structure is maximally adapted for mounting an immune response and efficiently filtering the flowing lymph.

Shape and sizeBean-shaped structures ranging from 0.5 to 1.5 cm in size.

EfficiencyBiological filters that clear 95–99% of antigens from the lymph.

T lymphocytesThe dominant population, accounting for about 60% of the organ's cells.

B lymphocytesMake up approximately 40% of the node's lymphocytic composition.

Macroscopic Anatomy and Topography

Lymph nodes are located along the course of lymphatic vessels, most commonly grouped into regional chains. Their localization is physiologically adapted:

In mobile parts of the body (popliteal, inguinal, axillary, cervical) — muscle contractions during movement assist in pumping lymph through the nodes.
At the hila of internal organs (lung roots, mesentery, hepatic and splenic hila) and along major blood vessels of the abdomen and pelvis.

Lymph nodes have a distinct polarity. On the convex side, afferent lymphatic vessels enter the organ. On the opposite, concave side lies the hilum: efferent lymphatic vessels and veins exit here, while arteries and nerve trunks enter.

General Histological Architecture

The organ's architecture includes a stromal framework and parenchyma. The stroma is of local origin (developing from embryonic mesenchyme) and consists of dense fibrous connective tissue. It forms the outer capsule and inward septa known as trabeculae.

The intertrabecular space is filled by the parenchyma, which is divided into three anatomical zones (from periphery to center):

Cortex (dark on histological sections).
Paracortex (intermediate zone).
Medulla (lighter central part).

Lymphoid tissue (clusters of immune cells) and a system of lymphatic sinuses are present in all zones of the parenchyma. Reticular tissue, consisting of specialized cells and fibers, serves as the microenvironment (stroma) for these cells.

Immunological Specialization (T and B Zones)

Lymphoid tissue is organized differently depending on the zone and is populated by cells originating from various primary lymphoid organs.

B-dependent zones: Located in the cortex as rounded lymphoid follicles (nodules) and in the medulla as medullary cords. These zones are populated by B lymphocytes originating from the red bone marrow.
T-dependent zone: Occupies the paracortical area, where lymphoid tissue is distributed diffusely. T lymphocytes migrate here from the thymus (comprising about 40% T helper cells and 20% cytotoxic T cells).

Notably, this zonation is not absolute. Cells continuously migrate, which facilitates essential contact-dependent interactions between T and B lymphocytes to initiate antigen-dependent differentiation.

System of Lymphatic Sinuses

Sinuses are intra-organ channels for lymph flow that function as highly efficient biological filters. Lymph flows sequentially through the following pathway: afferent vessels → subcapsular (marginal) sinus → perifollicular (trabecular) sinuses (between trabeculae and follicles) → medullary sinuses (between trabeculae and medullary cords) → efferent vessels.

The walls of the sinuses are lined with reticuloendothelial («gateway»)/ cells featuring gaps that allow free migration of lymphocytes. Within the sinus lumen, branching reticular cells form a three-dimensional network («sieve») that mechanically slows down lymph flow, while resident and free macrophages phagocytose antigens.

Mnemonic

B lymphocytes «love» extremes: they reside at the very periphery (cortical follicles) and the very center of the organ (medullary cords). T lymphocytes occupy the intermediate territory — the paracortical zone.

Frequently asked questions

Which specialized vessels facilitate the migration of lymphocytes from the blood into the paracortical zone?

Specialized postcapillary venules facilitate the migration of lymphocytes from the bloodstream into the paracortical zone.

They are classified as high endothelial venules (HEVs). The morphological feature of these vessels is that their endothelial cells are cuboidal or columnar rather than flat. Transendothelial migration (diapedesis) occurs precisely through the walls of these venules, allowing cells to exit the bloodstream into the lymph node tissue, after which T lymphocytes settle in the deep layers of the cortex.

What types of antigen-presenting cells are present in lymphoid follicles (nodules)?

Lymphoid follicles contain the following cell populations:

Macrophages — located within the germinal center and also found in the mantle zone (corona) of the follicle.
Dendritic cells — part of the cellular population of the germinal center.
Follicular dendritic cells — essential for the formation of follicular dendritic networks and lymphoid follicles, requiring lymphotoxin signaling.

Why are many lymph nodes located in mobile parts of the body?

Localization in such areas (e.g., popliteal or axillary regions) has a physiological purpose: muscle movement in these areas facilitates the mechanical pumping of lymph through the intra-organ sinus system.

Are the T and B zones of the lymph node strictly isolated from each other?

No, the zonation is relative. T cells can be found within B zones and vice versa. This migration is critically important for intercellular cooperation during the immune response.

How does lymph enter and leave the node?

Lymph enters via afferent vessels piercing the convex side into the marginal sinus, and leaves the organ via efferent vessels exiting the concave region, known as the hilum.

Go deeper

Response characteristics and localization of B1 lymphocytes (a population with a simplified response type).
Statistics of lymphocyte distribution across central and peripheral organs.
The role of the embryonic liver as a source of hematopoietic stem cells for lymph node development.
Mechanisms of antigen-dependent differentiation and cellular cooperation within the parenchyma.