Anatomical structure
The outer surface of a lymph node is covered by a tough connective tissue capsule. Septa, known as trabeculae, extend inward from the capsule into the parenchyma, creating the structural framework of the organ.
The angioarchitecture of the node's vascular bed is strictly polarized:
- Convex part: afferent lymphatic vessels empty here. They pierce the capsule and trabeculae, delivering lymph from drained peripheral tissues.
- Concave part (hilum): the site of entry for arterial vessels and exit for venous vessels. This is also where efferent lymphatic vessels originate, through which filtered lymph and lymphocytes leave the node.
Histological zones
A subcapsular sinus lies directly beneath the node capsule. The entire internal tissue is divided into clearly defined functional zones that ensure proper cell interaction:
- Outer cortex (B-zone). B lymphocytes are localized here, aggregated into rounded structures known as lymphoid follicles.
- Paracortical zone (T-zone). Located deeper than the follicles, surrounding postcapillary venules. It serves as a niche for T lymphocytes. Notably, this zone is thymus-dependent: in the absence of the thymus, it does not form.
- Mixed zone. The region at the border between the follicles and the paracortex. This is the exact site where the T–B cell contact, critical for the immune response, takes place.
- Medulla. Composed of medullary cords and separating medullary sinuses. During an immune response, plasma cells concentrate within the cords.
Lymphoid follicles and dendritic cells
Cortical follicles are of two types. Primary follicles have a uniform structure and are typical of a resting node. Secondary follicles appear in response to an antigen; they develop germinal centers and recruit T cells.
Follicular dendritic cells (FDCs) are responsible for follicle organization. Their ability to secrete survival factors in adults is maintained through continuous contact with B lymphocytes. Note: FDCs are unrelated to myeloid (bone marrow-derived) dendritic cells.
In contrast, true bone marrow-derived dendritic cells—interdigitating cells—function in the T-zone. They are named for their ability to wrap around T lymphocytes with cytoplasmic extensions (like interlaced fingers). They carry MHC molecules with autologous peptides on their surface, the recognition of which maintains the survival of naive T cells.
Embryogenesis and molecular mechanisms
Secondary lymphoid organs form primarily during embryogenesis (unlike mucosal structures, which can develop postnatally). In mice, this process occurs between the 9th and 17th days of development.
The basis of formation is a crosstalk between bone marrow-derived lymphoid tissue inducer cells (LTIC) and stromal precursor organizer cells (LTo).
- On the surface of LTIC, membrane lymphotoxins (LTαβ) and RANKL factor are expressed.
- They bind to TNFR family receptors on stromal cells.
- The stroma receives the signal and begins to synthesize IL-7 and chemokines (CXCL13, CCL19, CCL21).
- These factors attract mature lymphocytes, causing their spatial segregation into zones and forming the follicle primordium.
Briefly: answers to topical questions
What types of macrophages are present in the sinuses of a lymph node, and what are their functions?
Two types of macrophages are present in the lymph node sinuses: fixed and wandering.
- Fixed macrophages (resident) are located in the sinus walls.
- Wandering macrophages (free) are located within the sinus lumens.
Their primary function is phagocytosis: they destroy foreign bodies, antigens, and microorganisms, ensuring biological filtration and purification of the lymph flowing through the node.
Through which vascular structures do lymphocytes cross from the blood into the lymph node parenchyma?
Lymphocytes migrate from the blood into the lymph node parenchyma via specialized high endothelial venules (HEVs). These vascular structures are localized in the paracortex (thymus-dependent zone) of the node. The process of cells exiting the bloodstream into the tissue (extravasation) occurs via lymphocyte migration between endothelial cells. The high endothelium specifically attracts naive lymphocytes by secreting chemokines.
What is the sequence of lymph flow through the lymph node sinus system?
Lymph flows through the lymph node sinus system in a strict sequence from afferent to efferent vessels. Lymph passes through the following compartments:
- Subcapsular sinus (marginal sinus) — the initial entry point for lymph, located beneath the node capsule.
- Perifollicular sinuses (trabecular or interfollicular sinuses) — located in the cortex between trabeculae and follicles.
- Medullary sinuses — localized in the medulla between trabeculae and medullary cords.
- Hilar sinus — located at the hilum of the node, where lymph collects into efferent lymphatic vessels.
To keep the vessels straight: Afferent Attack the node (enter from outside, bring in antigens), Efferent Evacuate (exit from the hilum).
Class and exam questions
How do T lymphocytes find their way specifically to the paracortical zone?
Stromal and interdigitating cells of this zone secrete the chemokines CCL19 and CCL21. Naive T cells express the CCR7 receptor for these chemokines, allowing them to follow the chemical gradient directly into the T-zone.
How do follicular dendritic cells differ from interdigitating cells?
FDCs are located in the B-zone (follicles) and are not of myeloid origin. Interdigitating cells reside in the T-zone, originate from the bone marrow, and serve as classic antigen-presenting cells bearing MHC molecules.
What is the main difference in the blood and lymph supply between the spleen and lymph nodes?
The spleen monitors the blood flow and completely lacks afferent lymphatic vessels, whereas lymph nodes actively receive inflowing lymph.
What else to study on this topic
- The phenotype of lymphoid tissue inducer cells (LTICs) and the role of RORγt and Id2 factors
- Structure and functions of the spleen as a hematogenous barrier
- Mechanisms of homeostatic regulation of lymphocyte numbers in the peripheral system
- The role of interleukin-7 (IL-7) in naive T-cell survival
- Differences between naive and memory cells in the normal state
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