STUDIES ON LYMPHOCYTES

Continuous extracorporeal irradiation of the circulating blood (ECIB) of from 3 to 501/2 hr duration was used to study in the calf the differential depletion of lymphocytes from spleen, lymph nodes and thymus as compared to blood and thoracic duct lymph. The cell content of tissues was measured by planimetry and/or test point analysis. Lymphocyte depletion by ECIB from various lymphoreticular organs, and from different areas within a given organ, was less than in the circulating blood or the thoracic duct lymph and varied from one site of a lymphoreticular organ to another. The degree of depletion with time followed an exponential function with at least two components. The first component corresponded to a relatively rapid fall and the second to a very slow reduction in lymphocyte content. The former is related to the elimination of an easily mobilizable pool of lymphocytes while the latter corresponds to a more sessile mass of lymphocytes which exchange with blood lymphocytes very slowly. Elimination of the easily mobilizable pool of lymphocytes by ECIB from all tissues studied was observed within 10–15 hr, indicating that the rate of exchange with blood is similar for this group of cells in various lymphoreticular tissues. The size, however, of the easily mobilizable vs the more sessile pools of lymphocytes may vary considerably, the best estimates for the former being as follows (in per cent of total lymphocyte mass): lymph node medulla, less than 10%; lymph node cortex plus paracortical zone, 18% (depletion mainly paracortical); red pulp of the spleen, 37%; densely populated white pulp of the spleen, 55%; and loosely populated white pulp of the spleen, 60%. In comparison, the approximate fractions of lymphocytes originating fromthe easily mobilizable pools in various lymphoreticular tissues plus the cells already circulating a t the onset of EClB correspond to 64% for the thoracic duct lymph and 78% for the circulating blood respectively. These findings are discussed in relation to production, recirculation and life span of lymphocytes, and immune reactions.     

Radiosensitivity of T and B lymphocytes. IV. Effect of whole body irradiation upon various lymphoid tissues and numbers of recirculating lymphocytes.

Groups of 10-week-old female CBA/J mice were exposed in whole body fashion to 0,5,50, and 500 rads and sacrificed in serial fashion 1,3,5,7,9,15, and 30 days after irradiation for morphologic evaluation of thymus, spleen, lymph node, and Peyer's patch, and assessment of the relative numbers of thymus-derived (T) and bone marrow-derived (B) cells in these tissues. The absolute and relative numbers of recirculating T and B cells mobilizable by thoracic duct cannulation were also determined and compared with similar determinations with respect to peripheral blood lymphocytes. B cell depletion occurred more quickly and was more pronounced in spleen and lymph node than T cell depletion at all three exposure doses. Depletion of T and B cells was roughly equal in peripheral blood and thoracic duct lymph. When present, regeneration of the T cell component occurred more rapidly than did B cell restoration. The latter often was incomplete at the time of the final sacrifice (day 30). PHA-responsive and Con A-responsive cells also appeared to differ with respect to the kinetics of cell death after whole body irradiation.     

Mathematical Modeling Reveals Kinetics of Lymphocyte Recirculation in the Whole Organism

The kinetics of recirculation of naive lymphocytes in the body has important implications for the speed at which local infections are detected and controlled by immune responses. With a help of a novel mathematical model, we analyze experimental data on migration of ⁵¹Cr-labeled thoracic duct lymphocytes (TDLs) via major lymphoid and nonlymphoid tissues of rats in the absence of systemic antigenic stimulation. We show that at any point of time, 95% of lymphocytes in the blood travel via capillaries in the lung or sinusoids of the liver and only 5% migrate to secondary lymphoid tissues such as lymph nodes, Peyer''s patches, or the spleen. Interestingly, our analysis suggests that lymphocytes travel via lung capillaries and liver sinusoids at an extremely rapid rate with the average residence time in these tissues being less than 1 minute. The model also predicts a relatively short average residence time of TDLs in the spleen (2.5 hours) and a longer average residence time of TDLs in major lymph nodes and Peyer''s patches (10 hours). Surprisingly, we find that the average residence time of lymphocytes is similar in lymph nodes draining the skin (subcutaneous LNs) or the gut (mesenteric LNs) or in Peyer''s patches. Applying our model to an additional dataset on lymphocyte migration via resting and antigen-stimulated lymph nodes we find that enlargement of antigen-stimulated lymph nodes occurs mainly due to increased entrance rate of TDLs into the nodes and not due to decreased exit rate as has been suggested in some studies. Taken together, our analysis for the first time provides a comprehensive, systems view of recirculation kinetics of thoracic duct lymphocytes in the whole organism.     

The influence of strong transplantation antigens (Ag-B) on lymphocyte migration in vivo.

The tissue localization of syngeneic thoracic duct lymphocytes was compared to that of allogeneic cells in four rat strain combinations differing at the Ag-B locus (HO → DA, DA → HO, AO → HO, HO → AO). Dual isotope labeling with [³H]uridine and [¹⁴C]uridine was applied in order so that the distribution of allogeneic and syngeneic cells could be followed in one recipient. During the first couple of hours after iv injection, allogeneic lymphocytes usually migrated as easily into the various tissues as did syngeneic cells. However, after 24 and 48 hr, a reduced amount of label associated with allogeneic cells was often measured in the tissues. This reduction differed in magnitude in the different strain combinations and was most pronounced in the lymph nodes. A reduced number of allogeneic cells also appeared in the thoracic duct. By contrast, no reduced localization of allogeneic lymphocytes was measured in the draining popliteal lymph nodes late after sc injection. In preimmunized animals allogeneic cells were rapidly removed from the blood and therefore failed to localize in the lymphoid tissues. Furthermore, the lymph node localization of allogeneic cells was more like that of syngeneic cells in splenectomized rats, as well as in irradiated recipients (when the irradiation was given shortly before cell transfer). It is concluded that transplantation antigens play no essential role in the interaction between recirculating lymphocytes and the venous endothelium at the sites where the large-scale physiological emigration of the cells takes place (the HEVS of the lymph nodes and the marginal zone vessels of the spleen). The elimination of allogeneic cells is found later; it probably takes place in the lymph nodes and spleen. Possible mechanisms responsible for this rapid removal of allogeneic lymphocytes in nonimmunized recipients are discussed.     

Homing of germinal-center cells into germinal centers of lymph node via afferent lymphatics

Summary Affinity of lymphoid cells for the microenvironment of germinal centers (GC), as detectable in transfer experiments by rapid homing in spleen GC from the blood, is a capacity expressed by only a subset of lymphoid cells, in particular by those constituting a GC. However, when introduced into the blood stream, these cells do not home into GC of lymph nodes and gut-associated lymphoid tissues. To investigate further this homing inability for high endothelial venule (HEV)-containing lymphoid tissues, GC cells isolated from donor rabbit appendix were labeled in vitro with ³H-leucine and injected into an afferent lymph vessel of recipient popliteal lymph nodes. Draining lymph nodes were removed 15 min to 24 h after cell administration and prepared for radioautography. For reference, the migration of cells isolated from Peyer's patches and thoracic duct lymph was also studied. By use of appendix GC cells, large numbers of labeled cells were found to migrate into GCs of the outer cortex centripetally, i.e., from the subcapsular sinus through the lymphocyte corona into the GC proper. The same was observed for cells from Peyer's patches, although in smaller numbers. Thoracic duct lymphocytes were only localized in the lymphocyte corona and the deep cortex. Thus, appendix GC cells and a subpopulation of cells from Peyer's patches can reach lymph node GC, but only when administered intralymphatically. We conclude that cells expressing affinity for the GC microenvironment do so for both spleen and lymph node GC, but do not have the capacity to interact with the wall of HEV; its implication for the understanding of the dynamics of a GC reaction is discussed.Abbreviations GC germinal center - GCC germinal-center cells - AGCC appendix germinal-center cells - GCPC germinal-center precursor cells - GCSC germinal-center seeking cells - HEV high endothelial venules - SRBC sheep red blood cells - PP Peyer's patch - TDL thoracic duct lymphocytes - NCS newborn calf serum - PBS phosphate-buffered saline - PNA peanut agglutinin - LN lymph node - LC lymphocyte corona - DC deep cortex unit     

Freezing of rat lymphocytes. 2. Distribution and circulation of frozen-thawed 51Cr-labelled cells

Radioactive labelled synegenic fresh and frozen-thawed rat lymphoid cells were injected into groups of animals and their distribution in liver, spleen, lung, lymph nodes, and thoracic duct recorded. Principally the same pattern of distribution was demonstrated after injection of both fresh and frozen-thawed cells. However, more radioactivity was recovered from the liver and less from lymph nodes and thoracic ducts in recipients of frozen-thawed than of fresh cell preparations. This is caused by some damage to the cells in the freeze-thaw process.     

Cytological composition of lymph in fever reaction

This work deals with an investigation into the lymph cytologic composition of thoracic lymph duct of rabbits in fever reaction (FR) of various duration. FR was accompanied by quantitative and qualitative shifts in lymph cytologic composition. There was an alternative rise and fall of the leucocyte number in the first hours of fever. The number of little and medium leucocytes decreased while the number of eosinophiles, insufficiently differentiated cells-blasts, large lymphocytes, prolymphocytes increased. Our investigations revealed a significant role played by the lymphatic system in lymphoid cells mobilization in FR, which is evident by a considerable lymphocyte number gaining entrance to the blood through thoracic duct.     

Physiology of B cells in mice with X-linked immunodeficiency. I. Size, migratory properties, and turnover of the B cell pool

In terms of certain immune functions and density of surface IgM, B cells from xid mice are often viewed as the equivalent of the immature (Lyb-5-) B cell subset of normal adult mice. In this paper we examine xid B cells with regard to certain physiologic functions, including homing to the lymphoid tissues, recirculation, and turnover. Xid mice were found to possess about one-third of the total number of B cells found in normal mice. This applied irrespective of whether one examined the spleen, lymph nodes, or outputs of B cells in thoracic duct lymph. In terms of migration to spleen, lymph nodes, and Peyer's patches, capacity to recirculate from blood to thoracic duct lymph, and turnover, xid B cells proved to be indistinguishable from normal spleen or thoracic duct B cells. Within these parameters, most xid B cells closely resemble the normal mature long-lived population of B cells residing in the recirculating pool of normal mice. Because xid B cells are functionally quite different from normal mature B cells, it seems reasonable to view xid B cells as an abnormal population not represented in normal mice.     

Natural killer cell activity in the rat. V. The circulation patterns and tissue localization of peripheral blood large granular lymphocytes (LGL)

Large granular lymphocytes (LGL) and T cells were separated from blood by centrifugation on discontinuous gradients of Percoll, were labeled with [3H]uridine or [111In]oxine, and were injected i.v. into syngeneic euthymic or athymic nude rats. The tissue distribution of these labeled cells was monitored for up to 24 hr after transfer by scintillation counting of tissue homogenates and autoradiography of tissue sections. In normal euthymic rats, the main sites of LGL localization were the alveolar walls of the lungs and spleen red pulp; however, they were not detectable in the major traffic areas of T lymphocyte recirculation, the spleen white pulp, and lymph nodes. Furthermore, the density of labeled LGL was very low in the small intestine, thymus, kidney, and liver, although on a per-organ basis, about 10% of the injected radioactivity was found in the liver by 24 hr post-injection. When 111In-labeled LGL were injected i.v. into rats with an indwelling thoracic duct cannula, they completely failed to enter the thoracic duct lymphocyte (TDL) population over an observation period of 6 days. This finding was markedly different from the results obtained with T cells and was consistent with the lack of natural killer and antibody-dependent cellular cytotoxicity activity observed among TDL, even in rats pretreated with the biological response modifier, poly I:C. LGL in athymic nude rats also failed to recirculate between blood and lymph. However, in contrast to normal euthymic animals, a significant increase in the localization of radiolabeled LGL to lymph nodes was observed in nude rats between 30 min and 24 hr. Taken as a whole, these findings define the areas within the lungs and spleen in which blood LGL normally localize, and clearly demonstrate that LGL do not normally recirculate between blood and lymph.     

Circulating T and B lymphocytes of the mouse. I. Migratory properties

Studies on the identity of thoracic duct lymphocytes (TDL⁴) from normal and T cell-depleted mice indicated that as many circulating B lymphocytes were produced by healthy T cell-depleted mice as normal mice. Proportions of T and B cells from the thoracic duct of CBA mice changed markedly during the first 4 days of drainage from 82% T cells and 16% B cells at 12 hr to approximately equal proportions of both classes after 3 days. In absolute terms, T cells were mobilized rapidly by thoracic duct drainage and B cells very slowly. Histologically, this was reflected in a rapid depletion of the T cell-dependent areas of the lymphoid organs. The B cell-dependent areas, in contrast, became depleted of lymphocytes only after drainage for a week or more.The homing properties of circulating lymphocytes were investigated using TDL from normal and T cell-depleted mice as relatively pure sources of T and B cells, respectively. Four hours after injection of ⁵¹Cr-labeled T and B cells, a large proportion of both cell classes were found in the spleen. By 24 hr, many T cells had left the spleen and appeared in the lymph nodes. Such redistribution by B cells, however, was minimal.Intravenously injected T and B cells, labeled with tritiated uridine (³HU), localized specifically in the T and B cell-dependent areas, respectively, of the lymphoid tissues.³HU-labeled T cells were found to recirculate rapidly from blood to lymph. Labeled B cells, in contrast, recirculated only very slowly.     

Antigenic relationship between bone marrow lymphocytes cortical thymocytes and a subpopulation of peripheral T cells in the rat: description of a bone marrow lymphocyte antigen.

Populations of rat bone marrow lymphocytes (BML) consisting of approximately 90 percent, “tnull” cells were prepared by density gradient centrifugation, passage through a column of fine glass beads, and treatment with anti-T cell and anti-B cell serum plus complement. Antisera to these bone marrow lymphocytes were raised in rabbits. After absorption with RBC and peritoneal exudate cells, the anti-BML sera were found by immunofluorescence to react selectively with “null” cells in bone marrow, with cortical thymocytes, and with a cortisone-sensitive subset of T cells in blood and in spleen, possibly in red pulp. The antigen that is common to these cell types is designated the rat bone marrow lymphocyte antigen (RBMLA). Lymphocytes that are positive fur KBMLA are negative for another lymphocyte-specific heteroantigen, rat musked thymocyte antigen (RMTA). As shown previously, RMTA is present on medullary thymocytes and ou cortisone-resistant T cells in white pulp of spleen, paracortex of lymph node and thoracic duct lymph. It is postulated that two developmentally and functionally distinct lines of T cells exist in peripheral lymphoid tissues of the rat, one derived from cortical thymocytes and one derived from medullary thymocytes. It is further postulated that the “null” population of bone marrow lymphocytes contains the lymphopoietic stem cells from which these two lines of T cells originate.     

Are Actinomyces viscosus antigens B cell mitogens?

An extracellular heteroglycan (ECHG) and a sonicated cell supernatant (SCS) of Actinomyces viscosus Ny 1 induced strong lymphocyte proliferation. This was shown with spleen and thoracic duct cells form germfree rats and confirmed with cells from conventional "nude" mouse spleens. Spleen cells developed direct plaque-forming cells against densely coupled TNP-SRBC. The mitogenic property of ECHG was diminished considerably after mild alkaline hydrolysis for lymphocytes form rat spleens and was totally abolished for cells from "nude" mouse spleens. These results suggest that ECHG and SCS have B cell mitogenicity.     

Lymphocyte adhesion to cultured Peyer's patch high endothelial venule cells is mediated by organ-specific homing receptors and can be regulated by cytokines

Adhesion of lymphocytes to high endothelial venule (HEV) cells is the first step in the migration of these cells from blood into lymph nodes and Peyer's patches (PP). In the present study, we isolated and cultured HEV cells from PP of the rat and assessed their capacity to interact with lymphocytes. Flow cytometric analysis with a rat HEV-specific mAb KJ-4 revealed that greater than 90% of the cultured cells were stained by the antibody. Furthermore, confluent monolayers of PP HEV cells retained the capacity to support the adhesion of lymphocytes from spleen, thoracic duct, and lymph nodes but not binding of immature cells from thymus and bone marrow, which are deficient in cells capable of binding to HEV in vivo. In addition, intraepithelial lymphocytes that preferentially migrated into mucosal lymphoid tissues were also enriched in cells that adhered to the endothelial monolayers. The binding process required energy, was calcium-dependent, and could be inhibited by cytochalasin D, trypsin, and mixed glycosidase. Interestingly, pretreatment of PP HEV cells with rTNF, IFN-gamma, or granulocyte-macrophage CSF significantly increased the endothelial adhesiveness for thoracic duct lymphocytes in a time- and dose-dependent manner. In contrast, stimulation of lymphocytes with phorbol ester or TNF resulted in the rapid modulation of the surface expression of the PP homing receptor and decrease in lymphocyte binding to normal or TNF-stimulated HEV cells. The adhesion of lymphocytes to normal or cytokine-stimulated HEV cells can be blocked by pretreatment of lymphocytes, but not HEV cells, with the PP homing receptor-specific 1B.2.6 antibody. Taken together, these experiments provide strong evidence that the interaction between lymphocytes and cultured HEV cells are mediated by adhesive mechanisms that regulate lymphocyte entry into PP in vivo and that cytokines can promote HEV adhesiveness for lymphocytes through increased expression of organ-specific ligands on HEV cells.     

Virus-induced alterations of lymphoid tissue. IV. The effect of Newcastle disease virus on the fate of transfused thoracic duct lymphocytes

⁵¹Cr-labeled thoracic duct lymphocytes were briefly incubated at 4 °C with Newcastle disease virus (NDV) and then infused into syngeneic rats. Virus diverted the homing of many donor cells from lymph nodes and spleen to the liver. Evidence was obtained suggesting that some NDV-treated lymphocytes initially trapped in the liver subsequently migrated into the lymph nodes. The results imply that NDV transiently interrupts the normal route of lymphocyte migration. Alterations in lymphocyte distribution were mediated by attachment of virus to the cell surface and were the same as those induced by incubating lymphocytes with V. cholera neuraminidase before infusion. It appears that reactions involving 2–3′ and/or2–8′ linked sialyl residues on the surface of recirculating lymphocytes can markedly affect their distribution in the body.     

A monoclonal anti-HEBFPP antibody with specificity for lymphocyte surface molecules mediating adhesion to Peyer's patch high endothelium of the rat

Cell surface molecules involved in lymphocyte adhesion to high endothelial cell venules (HEV) of Peyer's patches (PP) have been studied in the rat by using a mouse monoclonal anti-HEBFPP (1B.2) antibody. We previously showed that rat thoracic duct lymph contains a high endothelial cell binding factor termed HEBFPP, which in vitro blocks lymphocyte binding sites of HEVPP but not HEVLN. Monoclonal 1B.2 antibody was produced by fusing P3U1 myeloma cells with spleen cells of a mouse immunized with this material. Immunoprecipitation studies with 125I surface-labeled rat thoracic duct lymphocytes (TDL) showed that the antibody recognized an 80-kilodalton protein. This antigen was present in the majority of TDL, spleen, LN, and PP cells but was found on few (5 to 10%) thymus and bone marrow cells (indirect immunofluorescence). Treatment of TDL with 1B.2 antibody blocked their ability to bind in vitro to HEVPP; antibody treatment did not interfere with TDL adhesion to HEVLN. Analysis of 1B.2 antigen isolated from lymph and detergent lysates of TDL by antibody-affinity chromatography showed that this material had the capacity to block lymphocyte binding sites of HEVPP but not HEVLN. In contrast, material with such blocking activity was not isolated from detergent lysates of thymocyte, a population deficient in HEV-binding cells. The results indicate that the 1B.2 antigen is a component of the lymphocyte surface recognition structure mediating adhesion to HEVPP and provide further evidence that distinct adhesion molecules of rat TDL mediate interaction with high endothelium of LN and PP.     

应激抑制淋巴细胞转化的时间效应

本研究吵缚方法使大鼠接受应激刺激,然后分别取大鼠应激３、６、１２、１８ｈ和解除束缚后１２、２４、４８、７２、９６ｈ的淋巴结、脾脏提取物和血清,与刀豆素Ａ同时加入正常大鼠肠系膜淋巴结细胞悬液中育７２ｈ后用噻唑蓝（ＭＴＴ）比色分析法检测肠系膜淋巴结细胞的转化,来应激抑制淋巴细胞转化作用的出现和消失过程。结果如下：（１）应激３和６ｈ大鼠的淋巴结、脾脏提取物和血清对淋巴细胞的转化都没有明显的影响;（２）应     

3H-uridine incorporation as a T lymphocyte indicator in the rat

Although about 70% of rat thoracic duct small lymphocytes labeled readily in vitro with ³H-uridine, only 3–38% of peritoneal exudate lymphocytes labeled. Since exudate cells are mostly B lymphocytes, ³H-uridine in concentrations used were presumed to label the T lymphocyte. Percentages of small lymphocytes that labeled in cell suspensions from various tissues were consistent with other estimates of T cells in those sources: 74.7% in thoracic duct, 70.2% in blood and 65.6% in spleen. When lymphopenia was induced by polyethylene ³²P strips applied to the spleen, a procedure that depletes mostly small recirculating lymphocytes, both labeled (T) and nonlabeled (B) cells were depleted in similar time sequence. Both cell types recovered at a similar rate after the spleen strips were removed. Induction of peritoneal inflammation by PPD in tubercle-bacilli immune rats caused an enhanced lymphocytic exudation but no increase in percentage of labeled (T) lymphocytes.The defect in ³H-uridine incorporation that characterizes the rat B lymphocyte seemed to be relatively specific for that RNA precurser; ³H-cytidine labeled the majority of lymphocytes in peritoneal exudate.     

Morphology of germ-free piglets

The postnatal ontogeny of primary (thymus) and secondary (spleen, lymph node, lingual tonsil) lymphoid tissues was studied in germ-free colostrum-deprived piglets up to age of 68 days. The thymus, which is morphologically fully developed by the end of gestation, showed no significant differences in the germ-free and conventional state. In germ-free piglets, slow development of periarteriolarly organized lymph follicles occurred in the spleen up to the end of the observation period. As distinct from the conditions in the spleen of conventional animals, the presence of a large number of pyroninophilic cells was not observed in germ-free piglets and no germinal centres were found. A similar situation was seen in the mesenteric lymph nodes, in which, in conventional piglets, cells belonging to the plasmacyte series, as well as the germinal centres, proliferate by the 13th day. Differences were also found in the organization of the follicular lymphoid tissue in the wall of the terminal ileum. In germ-free piglets, the lymph follicles increased only very slowly in size during the observation period and germinal centres were absent, while in conventional piglots germinal centres were present from the 12th day. The view is expressed that the intestinal lymphoid tissue ought rather to be classified as peripheral lymphoid tissue.     

The migration of lymphocytes across specialized vascular endothelium. IV. Prednisolone acts at several points on the recirculation pathways of lymphocytes

Radioactively labelled thoracic duct lymphocytes from syngeneic rat donors were injected iv into recipients which had been given a continuous iv infusion of prednisolone at 1 mg/hr for 15–18 hr previously. The tissue distribution and recirculation into lymph of the labelled lymphocytes were compared quantitatively in the prednisolone-treated and control recipients by scintillation counting and autoradiography. The most prominent effect of prednisolone was to retard recirculating lymphocytes within the tissues to which they are normally distributed by the blood, namely the bone marrow, the spleen, and the lymph nodes. Although lymphocyte traffic was almost completely frozen by prednisolone, recirculating lymphocytes were not killed. A second effect of prednisolone was to impair the influx of lymphocytes from the blood into lymph nodes. Different groups of lymph nodes varied in the extent to which prednisolone inhibited the entry of lymphocytes, and previous antigenic stimulation completely exempted lymph nodes from this inhibition. Lymphocytes took a longer time to cross the walls of high endothelial venules in the lymph nodes of prednisolone-treated rats. A third effect of prednisolone was to increase the rate at which lymphocytes entered the bone marrow from the blood by crossing sinusoidal endothelium.     

The lymphatic system in body homeostasis: physiological conditions

The lymphatic system is an organized network composed of functionally interrelated lymphoid tissue, and transportation pathways of tissue fluid/lymph and lymphoid cells. Its main components are 1. migrating dendritic cells, macrophages and lymphocytes, organized lymphoid tissue such as lymph nodes, thymus, spleen, bone marrow, and lymphoid tissue in gut and lungs, liver lymphoid cells, and the dendritic cell network of nonlymphoid organs; 2. vessels (intercellular space, lymphatics, and perivascular spaces); 3. fluids (tissue fluid and lymph). The lymphatic system can be divided into the following compartments: peripheral (from the interstitial space to and within the nearest lymph node), and central (efferent lymphatics, cysterna chyli, and thoracic duct, all lymphoid organs). Organs and tissues with the most active afferent arm of the lymphatic system are skin, gut, and lungs. These are the body structures exposed to the external environment. All other nonlymphoid bodily tissues are also percolated by tissue fluid/lymph, and contain a network of dendritic cells and macrophages. Data obtained from normal human subjects on lymph composition and flow are presented. Future trends in lymphatic research are outlined.