Section 9: Lymphatic System
Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger,Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed
Plate 9.162 Palatine Tonsil
Plate 9.165 Lymph Node
Plate 9.170 Thymus
Plate 9.175 Spleen: Red Pulp
The lymphatic system is responsible for the protection of theindividual against a hostile external environment composed of foreignsubstances and organisms. Specific cells of this system candistinguish between ourselves specifically, "self," and seek out andinactivate or destroy many invasive foreign substances and organisms,"non-self." These cells are called immunocompetent cells, andthe entire system is frequently termed the immune system.Lymphoid tissue consists of reticular cells and their secretoryproduct, collagen Type 3 or reticular fibers, supporting masses oflymphocytes, macrophages, antigen-presenting cells, and plasmacells.
Lymphoid tissue is remarkably variable and may appear as a diffuseinfiltration into the lamina propria of mucous membranes or aswell-defined organs, such as the thymus. One classification oflymphoid tissue, based upon increasing structural/functionalcomplexity, is (1) diffuse lymphoid tissue, (2) lymph nodules, (3)tonsils, (4) lymph nodes, (5) thymus, and (6) spleen.
The simplest form, diffuse lymphoid tissue, is found throughoutthe body but, in particular, in the alimentary and respiratorytracts. Located in the lamina propria, it underlies the surfaceepithelium, surrounds mucosal glands and their ducts, and ischaracterized by a loosely organized mass of lymphocytes. The diffuseform of lymphocytic tissue grades into a more dense form, termedlymph nodules, of circumscribed masses of densely packedlymphocytes, which may be considered the basic structural unit oflymphoid tissue. Each nodule may contain a light staining centralarea, termed the germinal center, the presence of whichindicates a site of active lymphocyte proliferation. These "primary"nodules or lymph follicles are found in large numbers in the mucosaof the intestinal tract, notably in the ileum and vermiformappendix.
Groups of lymph nodules may be partially encapsulated as smallorgans with a definite lymphatic and blood vascular supply. Such isthe case in the tonsils, found in the pharynx. The three distincttonsillar masses include the palatine, lingual, and pharyngeal(clinically, the adenoids), which form an incomplete ring around theentrance to the throat. The palatine and lingual tonsils are coveredwith a stratified squamous epithelium, whereas the pharyngeal tonsilis covered with a pseudostratified columnar ciliated epithelium, withsome goblet cells characteristic of the nasopharynx. In adults, thepharyngeal tonsil is covered by a stratified squamous epithelium. Thepalatine and lingual tonsils have numerous epithelium lined pits,referred to as crypts, which may bifurcate. Surrounding thecrypts is a single layer of lymph nodules with germinal centers. Thepharyngeal tonsil does not possess true crypts but rather widenedducts of underlying glands. The epithelium covering the tonsils isextensively infiltrated by lymphocytes, plasma cells, andpolymorphonuclear leucocytes.
Lymph nodes are completely encapsulated ovoid structures, incontrast to the lymphatic tissue previously described, and are theimmunologic filters of the lymph. The capsule admits afferentlymphatic vessels containing valves that provide one-way flow intothe subcapsular sinus. The lymph circulates through sinuses locatedin the cortex (containing the lymph nodules) and the medulla(containing lymphatic cords), and leaves the node via larger butfewer efferent lymphatic vessels. These also contain valves andemerge from a specific region of the node, the hilus. Lymph nodes,which vary in size from 1 to 25 mm, receive their blood supply onlyat the hilus of the node. The arterial vessels enter both thetrabeculae formed from the capsular connective tissue and themedullary cords, and they regionally supply the node by giving offcapillaries; they continue to the cortex, where an arterial branchpenetrates each cortical lymph nodule and forms a capillary plexusaround the germinal center. From the capillary beds, blood is carriedby veins, which follow a pathway similar to the arteries, leaving thenode at the hilum along with efferent lymphatic vessels.
The thymus varies in size and undergoes structural alterationswith age. It undergoes rapid growth until the end of the second year,after which time the rate of growth slows until approximately thefourteenth year. After this, the thymus begins to involute ordecrease in size, and, gradually, the lymphatic tissue is largelyreplaced by fat and connective tissue. In old age, very little thymictissue may be present. The thymus consists of two lobes joined byconnective tissue. Each lobe contains many lobules, which are 0.5 to2 mm in diameter and which are incompletely separated from eachother. A lobule is composed of a cortex and a medulla, which sends aprojection to join with the medullae of adjacent lobules. The cortexconsists of lymphocytes, which are densely and uniformly packed,obscuring the sparse reticular framework. The cortex lacks lymphnodules. The medulla stains less intensely as a result of thinning ofthe concentration of lymphocytes, and it is here that reticular cellscan be recognized. Hassall's thymic corpuscles, located in themedulla, are diagnostic for identifying the thymus. The diameter ofHassall's corpuscles varies from 20 to 150 µm. The origin andnature of Hassall's corpuscles is unknown but may representdegeneration residue.
Arteries supplying the thymus follow the connective tissue septaand give off branches that enter the lobular cortex and break up intocapillaries, which supply the cortex. Epithelial reticular cellssequester developing lymphocytes and form a sheath coveringcapillaries and lymphatic vessels. The sheathing forms what is calledthe blood-thymus barrier, preventing antigen contamination ofdeveloping and programmed T lymphocytes. The blood-thymus barrier isnot found in the medulla, which appears to have a richer blood supplythan the cortex. The capillaries terminate in thin-walled veinslocated in the connective tissue septa along with arteries. Lymphaticvessels arise within the thymic lobule and join to form largervessels, which accompany the arteries and veins in the septa. Incontrast to lymph nodes, the thymus contains no lymph sinuses orafferent lymphatic vessels.
The spleen is the largest lymphatic organ in the body and is theimmunologic filter for the blood. Like the thymus, it has no afferentlymphatic vessels and no lymph sinuses. Splenic vessels enter andleave the spleen at the hilum of the organ and are located in thicktrabeculae, which extend inward from the capsule. The capsule andtrabeculae are composed of collagen and elastic fibers and somesmooth muscle fibers. A reticular fiber network and lymphocytes arefound between the trabeculae.
Sections of fresh spleen reveal two different regions, theso-called red and white pulps. The red pulp is traversed by a plexusof venous sinuses separated by lymphatic splenic cords. The venoussinuses contain tightly packed red blood corpuscles when they performa storage function. The white pulp is composed of compact lymphoidtissue arranged in spherical or ovoid aggregations around arterioles(central arterioles). These aggregations are called splenic,or Malpighian corpuscles, and bear a resemblance to lymphnodules.
The vascular supply is critical for an understanding of thespleen. The arteries enter at the hilus and are carried in, andbranch with, the trabeculae. Arterioles emerge from the trabeculaeand pass into the splenic parenchyma, where the adventitia of thearterioles is infiltrated by lymphocytes to form splenic corpuscles.These arterioles supply the capillaries for the white pulp andcontinue their course, lose their lymphatic investment, and enter thered pulp, where they subdivide into several branches calledpenicilli. These branches become smaller and aredifferentiated into three distinct regions: pulp arterioles, sheathedarterioles, and terminal capillaries. The nature of the terminationof these capillaries and their ultimate union with venous sinuses iscontroversial. A discussion of this point can be found incomprehensive textbooks of histology. The venous sinuses are linednot by endothelium but by specialized reticular cells, which arefixed macrophages. The reticular cells are encircled by reticularfibers. The venous sinuses unite to form pulp veins, which are linedby endothelial cells. The pulp, or collecting, veins enter thetrabeculae and leave the spleen at the hilum.
Lymphocytes and monocytes develop in both the red and white pulp,the primary source, however, being the white pulp. They migrate tothe white pulp to gain access to the venous sinuses. Although thespleen is not essential for life, it carries out several veryimportant functions, including (1) filtering the blood by removingfrom the circulation foreign particles and aging red blood corpusclesand leucocytes; (2) conserving and temporarily storing iron recoveredfrom hemoglobin of removed corpuscles; (3) storing normal red bloodcorpuscles within the splenic sinuses; (4) playing a key role inantibody formation; and (5) generating lymphocytes and monocytes,which enter the general circulation.
There are two classes of lymphocytes: T lymphocytes and Blymphocytes. These cells are functionally different but structurallysimilar, at least at the level of the light microscope. T lymphocytesare thymus-derived and are involved in cellular immunity, in whichthey interact with and destroy foreign or "non-self" cells. The Blymphocytes are involved with humoral immunity. These cells interactwith foreign substances, then differentiate into plasma cells andsynthesize and secrete immunoglobulins. The two immune systems areassisted by macrophages and certain other cells known asantigen-presenting cells (e.g., Langerhans cells of the skinand Kupffer's cells of the liver). Both the T and B lymphocytes havesubpopulations that play a role in the immune system. The major Tcell subgroups are the helper, suppressor, and killer cells. Helpercells are necessary in the initial antigen responses, especially togenerate IgG and IgA responses. The immune response has potentiallygood as well as harmful effects and should be modulated to prevent ahyperimmune response. The T-suppressor cell serves this purpose. TheT-killer cells are the effector cells of the thymus-dependent system.They combine with the antigen to initiate the cytotoxic mechanisms,which kill the invading organism. T cells are the major immune factorinvolved in the rejection of organ transplants and are theresponsible culprits in the process known as graft/host reaction. Inaddition, T cells are involved in the immune response to acid- fastbacteria, certain viral infections, and fungi. They are also the mainmediators of the immunopathological mechanisms in contactdermatitis.
Subpopulations of the B lymphocytes have not been as well definedas those of the T cells but are believed to exist on the basis ofsurface marker analysis. B cell products, the immunoglobulins, aredivided into five major classes, each of which is produced by adifferent cell line.
Plasma cells produce five kinds of immunoglobulins, which have thefollowing characteristics:
- IgG constitutes about 75 per cent of serum immunoglobulin, which provides binding sites for antigens. This immunoglobulin, produced by a mother, also provides protection for her newborn against infection because it can cross the placenta.
- IgA is found in colostrum, saliva, tears, and nasal, bronchial, intestinal, prostatic, and vaginal secretions. It is synthesized by the mucosal epithelial cells. Another type of IgA and associated proteins are synthesized by plasma cells located in the mucosa of the digestive, respiratory, and urinary tracts.
- IgM is important for early immune responses and may be bound to B lymphocytes, or it may circulate in the blood. The bound form (along with IgD) is a receptor for antigens, which leads to the differentiation of anti body- producing plasma cells. IgM can activate a group of plasma enzymes (complement) capable of lysing bacteria and other cells.
- IgE is secreted by plasma cells and attaches itself to basophils and mast cells. When the antigen that induced IgE synthesis and secretion is once again encountered, the basophils and mast cells release their stored histamine, heparin, leucotrienes, and eosinophil chemotactic factor, resulting in an allergic reaction. Leucotrienes are important compounds mediating allergic reactions, such as in asthma, which are produced by mast and perhaps, other cells.
- IgD is found on the surface membrane of B lymphocytes with IgM, but its function is uncertain.
Thus, of the immunoglobulins, IgM is considered the first line ofdefense. IgG has a long half-life and can cross the placenta, thus isideally suited for passive immunization. IgA protects mainly thesecretory surfaces (gastrointestinal tract and eyes) where there arenonvascular exposures to antigens and conditions that may interferewith the usual antibody activity, such as acid secretion, intestinalmotility, and proteolytic enzymes. IgE is important in the release ofpharmacologically active agents from mast cells and thus causesasthma and hayfever. It is also the major mechanism in theelimination of parasites. IgD is primarily a lymphocyte receptor, isthe strongest binding antibody, and is important in directing antigento B cell surfaces to accomplish initial immunization.
T lymphocytes that migrate into other lymphoid tissues are locatedin so-called thymus-dependent areas such as the paracortical zone oflymph nodes and periarterial sheaths of the white pulp of the spleen.The paracortical area is an ill-defined band or zone that liesbetween the cortex and medulla. T lymphocytes are long-lived andconstitute most of the lymphocytes in lymph and blood. B lymphocytesare located in the nodules of the spleen, lymph nodes, and lymphaticaggregations of the ileum (Peyer's patches).
When a microbe or a virus invades the body, white cells (includingneutrophils) are among the first of the body's defenses to attack theinvading organisms. White cells are short-lived scavengers andsurvive only a few days. Macrophages, however, are long-livedscavengers that engulf cellular debris and foreign matter.Macrophages display specific markers from the invading organisms ontheir surface known as antigens. Antigens signal helper T cells,which begin reproducing themselves. The helper T cells in turnproduce chemicals (interleukins) to activate B cells. The B cellsbegin reproducing themselves and mature into plasma cells. Plasmacells produce antibodies, which are specifically intended to destroythe invading organism either directly by binding to it or by makingit more vulnerable to macrophages and neutrophils. After the invaderhas been destroyed, suppressor T cells chemically notify B cells andhelper T cells to return to a dormant state.
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