Histological Methods for Studying Lymph Nodes
Different approaches are used to visualize the various structures of the organ:
- Standard hematoxylin and eosin (H&E) staining. This reveals the general architecture: the capsule, subcapsular and medullary sinuses, follicles (including the mantle zone and germinal center), paracortical zone, and medullary cords.
- Silver impregnation. Used to identify the stroma. Reticular fibers are argyrophilic (affinity for silver) and stain black-brown, forming a dense framework.
- Functional macrophage staining. Based on the in vivo administration of a vital dye to an animal. The dye accumulates inside typical macrophages located within the medullary sinuses and along their walls (so-called "littoral cells"). Meanwhile, the lymphoid cells of the medullary cords remain free of dye particles.
Paracortical Zone and Migration Pathways
The paracortical zone is evaluated first when studying the immune response because it lacks ordered structures like follicles (cells are arranged diffusely), yet critical early reactions unfold here.
The cell population is predominantly T lymphocytes. Macrophages and interdigitating dendritic cells (IDCs)—elements with long finger-like processes derived from lymph-borne dendritic cells—are also present.
A vital structure of this zone is the high endothelial venule (HEV). Their endothelial cells are so tall that they bulge into the lumen. Their surface features specific receptors that capture lymphocytes. This ensures massive cell migration from the blood into the node, allowing the entire cellular composition of the lymph node to completely renew within a few days.
Upon antigen arrival (via lymph, blood, or inside activated macrophages secreting interleukins IL-1 and IL-6), local inflammation develops in the paracortical zone, blood flow increases, and lymphocyte influx rises, clinically manifesting as lymph node enlargement and tenderness.
Lymphatic Follicles and B-Cell Response
Follicles drive the development of humoral immunity. They are divided into two types:
- Primary follicles. Found in fetuses and gnotobiotic animals (raised in sterile conditions). They are small, homogeneous, lack a germinal center, and are filled with resting B cells. In the postnatal period, they rapidly transform into secondary follicles.
- Secondary follicles. Have a complex structure. The periphery features a dark mantle zone composed of small lymphocytes, while the center contains a germinal (reactive) center of active proliferation.
The germinal center is divided into three zones: the dark zone (at the base, facing the paracortical zone), the light basal zone, and the light apical zone.
Antigen-dependent differentiation begins in the dark zone. Dendritic cells hold "IgM + native antigen" complexes on their processes. Incosomes—corpuscles carrying these complexes—break off from these processes. B lymphocytes recognize the antigen on incosomes, engulf it, and present peptides to $T_{h2}$ cells. This contact activates B cells: they begin actively dividing, and somatic hypermutation of immunoglobulin genes is initiated (at this stage, the cells are called centroblasts). Next, the cells migrate to the light basal zone, where they are designated as centrocytes.