Anatomical compartments

The lymph of mammals is classified as being peripheral, intermediate or central. Peripheral lymph is lymph which has not yet passed through a lymph node. Intermediate lymph is lymph contained in vessels between lymph nodes. Central lymph is lymph contained in the major lymph trunks like the intestinal, thoracic, right lymph duct or deep cervical duct. Central lymph does not pass through any lymph nodes before it enters the great veins at the root of the neck (Figure 2).

Peripheral lymph

True peripheral lymph has a very low content of white cells, usually well under 1000 mm iMost of these are small lymphocytes of which some 85% are T cells. However, peripheral lymph is distinguished by the presence of 'veiled' dendritic cells, which account for about 10% of the cellular content

(Figure 3). Their main function is probably the induction of T cell-mediated immunity.

Intermediate lymph

Intermediate lymph is populated by large numbers (up to 20 000 rnrrf *) of small lymphocytes, some 75% of which are T cells. Nearly all of these are recirculating lymphocytes that have extravasated from specialized postcapillary venules in the node. In rodents and humans, these specialized venules are recognizable in histological sections because of their cuboidal or 'high' endothelium (high endothelial venules, HEV) between which extravasating lymphocytes may be identified. This high endothelium is not seen in the lymph nodes of ruminants or athymic rats, even though the recirculation of lymphocytes is taking place. Lymphocyte extravasation in sheep proceeds via paracortical venules in lymph nodes. The function of these venules appears to be analogous to HEV in nonruminant species.

Hardly any of the dendritic cells from peripheral lymph appear in intermediate lymph. The majority are retained by the first node to which the lymph stream carries them. In this context, it should be remembered that more deeply seated nodes will receive two sorts of afferent lymph. One will be lymphocyte-rich lymph efferent from more peripheral nodes; the other will be genuine hypocellular, peripheral lymph formed in the adjacent tissues.

The lymphatic system in the gut has a rather special organization. There is an abundant extravasation of lymphocytes in the wall of the gut. This occurs diffusely throughout the length of the intestine, but also in macroscopic aggregates of subepi-

Figure 3 Phase-contrast photomicrograph (approximately x 1500) of small lymphocytes and 'veiled' dendritic cells in peripheral lymph from the leg of a sheep.

thelial lymphoid follicles that occur at intervals along the antimesenteric aspect of the small gut. These structures, the Peyer's patches, are believed to provide the B and T cells necessary for inducing immune responses which result in the production of IgA, the principal immunoglobulin in the secretion which protects all mucosal surfaces. In ruminants, the ileal Peyer's patch is also the site at which immature B cells undergo proliferation and the genetic rearrangements that endow them with the potential for Ig synthesis. In all mammalian species, these special features endow peripheral intestinal lymph with a content of lymphocytes that is about 50 times greater than that of peripheral lymph from any other organ. The lymph also contains substantial numbers of dendritic cells and, in postnatal animals whose guts have been colonized by bacteria, significant numbers of immunoblasts are also present. The regional nodes of the gut - the mesenteric nodes - are thus perhaps unique in receiving a supply of hypercellular afferent lymph that is already immunologically activated to some degree. It may be, however, that the same applies to those cervical nodes which receive lymph from Waldeyer's ring (i.e. tonsils, adenoids, etc.) but direct evidence is lacking.

There are other noteworthy features of the intestinal lymphatic system. The intestine is the site of the absorption of fatty acids from the diet, and these are processed into minute fat globules - the chylomicra -by the enterocytes which then discharge them into the lymph and give it its characteristic milky appearance.

Central lymph

Central lymph is merely a conglomerate of all the intermediate lymph contained in the various lymphatics that unite to form the major lymph trunks. For the practical purposes of immunology, thoracic-duct lymph is the most commonly collected example. It is as well to remember that the thoracic duct is the final common pathway of all lymph generated below the level of the diaphragm, and the majority of this is lymph from the intestine. In sheep, the small lymphocytes from intestinal lymph can be shown to recirculate preferentially through the gut, while those from nonintestinal lymph recirculate mainly through other tissues. At present there is no direct evidence that this distinction applies to humans and rodents. There is less uncertainty about the behavior of the large lymphocytes (immunoblasts) which are present in intestinal, and thus thoracic duct lymph. After being discharged into the blood they extravasare abundantly and rapidly in the lamina propria of the small gut. Many of them can be shown to transform into classical plasma cells which synthesize and

Figure 3 Phase-contrast photomicrograph (approximately x 1500) of small lymphocytes and 'veiled' dendritic cells in peripheral lymph from the leg of a sheep.

secrete antibody. Much of this is of the IgA class; that which is not immediately secreted into the lumen of the gut is swept up into the intestinal lymphatics, conveyed to the blood and transported rapidly into the bile by the active transcytotic vesicles in the hepatocytes.

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