Anatomy

Lymph nodes are ovoid encapsulated structures situated at regular intervals along the lymphatic channels. They neutralise, degrade or modify the antigens that are presented to them by the lymphatics before returning them to the blood. The immune response to antigens after birth determines the structure and composition of the node. Each lymph node has a fibrous capsule from which trabeculae extend into the parenchyma. The non-stimulated/minimally stimulated lymph node is composed of a reticulum meshwork supported by fibroblastic dendritic cells. The broad functional and anatomical divisions within a lymph node are the outer cortex and the inner medulla, which are sometimes visible on naked-eye examination (Figure 44.1 ). The cortex or the B zone contains pale-staining, densely packed aggregates of lymphocytes called primary follicles. These are separated from each other and the sinuses by smaller lymphocytes forming a mantle

Afferent lymphatics

Afferent lymphatics

follicles

Figure 44.1. Architecture of lymph node.

follicles

Figure 44.1. Architecture of lymph node.

of darkly staining cells, the mantle zone, and yet another zone of paler cells, the marginal zone. Deep to the cortex and between follicles is the paracortex or T zone, composed mostly of T cells mixed with histiocytes, interdigitating reticulum cells and Langerhan's cells. The paracortex also has the characteristic high endothelial venules that are involved in lymphocyte trafficking.

Following antigenic stimulation the cells of the primary B follicles become larger, acquire multiple nucleoli, divide and die. The germinal centres of the secondary follicles thus formed contain immunoblasts, centroblasts, centrocytes and tingible body macrophages. Antigenic stimulation also results in a paracortical T cell response with transformation to blast cells. The medulla contains large numbers of plasma cells, which are the terminally mature B cells.

The afferent lymphatics enter the lymph node through the convex surface on the cortex and drain into the subcapsular venous sinuses. The lymph is conveyed to the efferent lymphatics in the hilum by the intermediate and medullary sinuses.

Three lineages of lymphocytes are recognised in the lymph node, the B, T and NK (natural killer) cells. The B lymphocytes express surface and cytoplasmic immunoglobulins, which mediate humoral immunity. Large quantities of immunoglobulin are produced by the plasma cells. The T cells including the helper and suppressor subsets mediate cellular immunity. The two mechanisms of immunity are interdependent.

Immunophenotyping is essential in characterisation of lymphoid diseases and it is important to be familiar with the normal immunoarchitecture of the lymph node (Figure 44.2 ).

In general, the follicles stain strongly with B cell markers (CD19, CD20, CD 79a). The interfol-licular and paracortical regions express CD3 ( T cell marker ) predominantly. The developmental stages and immunophenotype of B and T lymphocytes are shown in Figure 44.3.

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