Two types of lymphatic vessels serve the lymph node:  Afferent lymphatic vessels convey lymph toward the node and enter it at various points on the convex surface of the capsule. Efferent lymphatic vessels convey lymph away from the node and leave at the hilum, a depression on the concave surface of the node that also serves as the entrance and exit for blood vessels and nerves. Note that activated lymphocytes, which remain in the lymph node to proliferate and differentiate, are carried to the node primarily by blood vessels. The supporting elements of the lymph node are:  capsule, composed of dense connective tissue that surrounds the node;
FIGURE 14.16 • Photomicrograph of aggregated nodules in the wall of the ileum. This low-magnification photomicrograph provides an example of aggregated nodules. The multiple lymphatic nodules (indicated by a dashed line) with visible germinal centers are typically found in the ileum. This accumulation of lymphatic tissue is known as a Peyer’s patch. The nodules originate in the lamina propria and extend into the submucosa of the ileum. 5.
 trabeculae, also composed of dense connective tissue, which extend from the capsule into the substance of the node, forming a gross framework; and  reticular tissue composed of reticular cells and reticular fibers that form a fine supporting meshwork throughout the remainder of the organ (Fig. 14.18). The reticular meshwork of lymphatic tissues and organs (except the thymus) consists of cells of mesenchymal origin and reticular fibers and ground substance produced by those cells.
Cells of the Reticular Meshwork
The reticular meshwork of the lymph node contains several types of cells that perform different functions in generating immune responses.
The cells of the reticular meshwork appear as stellate or elongated cells with an oval euchromatic nucleus and a small amount of acidophilic cytoplasm. These cells can take up dyes and colloidal materials. Using immunocytochemistry and transmission electron microscopy, several populations of cells have been identified.
 Reticular cells are indistinguishable from typical fibroblasts. These cells synthesize and secrete type III collagen (reticular fibers) and the associated ground substance that forms the stroma observed with the light microscope (Plate 38, page 481). Elongated cytoplasmic processes of these cells wrap around the bundles of reticular fibers, effectively isolating these structural components from the parenchyma of the lymphatic tissue and organs (Fig. 14.19). Besides their supporting role, they express surface molecules and produce substances that attract T cells, B cells, and dendritic cells.
 Dendritic cells (DCs) are unique bone marrow–derived APCs. DCs monitor the local environment for foreign substances that they then process and present to antigen-specific T cells. They are much more efficient in antigen presentation than other APCs and can present virtually any form of protein antigen on both MHC I and MHC II molecules. They express an exceptionally high level of MHC II and costimulatory molecules necessary for activation of T cells. In the lymph node, DCs are usually localized in T lymphocyte–rich areas.
 Macrophages are both phagocytic and antigen-presenting cells that express MHC I, MHC II, and costimulatory molecules. However, the expression levels of MHC II and costimulatory molecules are much lower than those of the dendritic cells, making them less efficient APCs. Instead, they have an immense capacity for endocytosis and digestion of internalized materials. The structure, microscopic characteristics, and functions of macrophages are described in Chapter 6, Connective Tissue.