Download Lymphatics and Immunity

Emma Gottlieb
February 10, 2011
Anatomy 1 Extra Credit
The Lymphatic System and Immunity
Introduction
An animal must defend itself against the many potentially dangerous viruses,
bacteria, and other pathogens it encounters in the air, in food, and in water. It must also
contend with abnormal body cells that may develop into cancer. Two major kinds of
defense have evolved to counter these threats. The first, called innate immunity, is
present before any exposure to pathogens and is effective from the time of birth. Innate
defenses are largely nonspecific, quickly recognizing and responding to a broad range of
microbes regardless of their precise identity. Innate immunity consists of eternal barriers
formed by the skin and mucous membranes, plus a set of internal cellular and chemical
defenses that combat infectious agents the breach the external barriers. Key players in
these internal defenses are macrophages and other phagocytic cells, which ingest and
then destroy pathogens.
The second major type of immunity is adaptive immunity. It develops only after
exposure to inducing agents such as microbes, abnormal body cells, toxins, or other
foreign substances. Acquired defenses are highly specific – that is, they can distinguish
one inducing agent from another, even though they differ only slightly. This recognition
is achieved by white blood cells called lymphocytes, which produce two general types of
immune responses. In the humoral response, cells derived from B lymphocytes secrete
defensive proteins called antibodies that bind to microbes and mark them for
elimination. In the cell-mediated response, cytotoxic lymphocytes directly destroy
infected body cells, cancer cells, and foreign tissue.
Lymphatic System Structure and Functions
The lymphatic system is a subsystem of the circulatory system in the vertebrate
body that consists of a complex network of vessels, tissues, and organs. The lymphatic
system helps maintain fluid balance in the body by collecting excess fluid
and particulate matter from tissues and depositing them in the bloodstream. It also helps
defend the body against infection by supplying disease-fighting cells called lymphocytes.
Functions of the Lymphatic System
The lymphatic system works in close cooperation with other body systems to
perform these important functions:
1.
The lymphatic system aids the immune system in destroying
pathogens and filtering waste so that the lymph can be safely returned
to the circulatory system. The immune system uses two lymphocytes,
T cells and B cells. Cytotoxic T cells, which are activated by various
cytokines, bind to and kill infected cells and cancer cells. B
lymphocytes differentiate into plasma and secrete antibodies.
2.
3.
To remove excess fluid, waste, debris, dead blood cells, pathogens,
cancer cells, and toxins from these cells and the tissue spaces
between them.
The lymphatic system also works with the circulatory system to
deliver nutrients, oxygen, and hormones from the blood to the cells
that make up the tissues of the body.
Lymphatic Vessels and Lymph Circulation
Lymph is conveyed from the tissues to the venous bloodstream via the lymphatic
vessels. On the way, it is filtered through lymphatic organs (spleen and thymus) and
lymph nodes. Pressure within the walls of the lymph vessels is lower than that in blood
vessels. Lymph flows more slowly than blood. The cells walls of the lymph vessels are
more permeable than those of capillary walls of blood vessels. Thus, proteins that may
have been delivered to the tissues by the blood stream, but that are too big to reenter the
capillaries, along with waste products and large proteins synthesized in the local tissue
cells, enter the lymphatic vessels for return to the bloodstream.
The fluid and proteins within the tissues begin their journey back to the
bloodstream by passing into tiny lymphatic capillaries that infuse almost every tissue of
the body. Only a few regions, including the epidermis of the skin, the mucous
membranes, the bone marrow, and the central nervous system, are free of lymphatic
capillaries, whereas regions such as the lungs, gut, genitourinary system, and dermis of
the skin are densely packed with these vessels.
Lymphatic Capillaries
The diameter of lymphatic capillaries is larger than that of blood capillaries. The
endothelial cells making up the walls of the capillaries have their ends to overlap which
makes the capillary able to let fluid in but not back out. Pressure outside of the lymphatic
capillaries causes cells to separate and allow interstitial fluid into the capillary. Once
inside the capillary, the fluid cannot get back out because the pressure is now greater in
the capillary and the cells pack together. Anchoring filaments are attached to the
lymphatic capillaries and their elastic fibers anchor the epithelial cells of the capillary to
the surrounding tissues. These are pulled when swelling occurs and more fluid has to be
allowed in.
The small intestines contain lacteals, which are specialized forms of lymphatic
capillaries. They carry dietary lipids into lymphatic vessels and then continue on to the
blood. Chyle is the liquid that includes these lipids because now the appearance is a
milky white instead of the normal clear, pale yellow color of lymph.
Lymph Trunks and Ducts
Once within the lymphatic system, the extracellular fluid, which is now
called lymph, drains into larger vessels called the lymphatics. These vessels converge to
form one of two large vessels called lymphatic trunks, which are connected to veins at
the base of the neck. One of these trunks, the right lymphatic duct, drains the upper right
portion of the body, returning lymph to the bloodstream via the right subclavian vein.
The other trunk, the thoracic duct, drains the rest of the body into the left subclavian
vein. Lymph is transported along the system of vessels by muscle contractions, and
valves prevent lymph from flowing backward. The lymphatic vessels are punctuated at
intervals by small masses of lymph tissue, called lymph nodes, that remove foreign
materials such as infectious microorganisms from the lymph filtering through them.
There are five main lymphatic trunks in the body – the lumbar, intestinal,
bronchomediastinal, subclavian, and jugular trunks. The lumbar trunks drain lymph
from free lower limbs, the wall and viscera of the pelvis, the kidneys, the adrenal glands,
and the abdominal wall. The intestinal trunk drains lymph from the stomach, intestines,
pancreas, spleen, and part of the liver. The bronchiomediastinal trunks drain lymph
from the thoracic wall, lung and heart. The subclavian trunks drain the free upper limbs.
After going through the lymph trunks, lymph travels to two main ducts, the
thoracic duct and the right lymphatic duct. The thoracic duct is a vascular structure
which recirculates lymph into the blood stream. It begins in the abdomen and tracks
alongside the aorta and esophagus to eventually join with the left brachiocephalic vein.
The thoracic duct begins as a dilation known as the cisterna chyli. This is the primary
duct that returns lymph to blood. The thoracic duct collects lymph from the left side of
the head, neck, and chest, the left free upper limb, and the entire body below the ribs. The
right lymphatic duct is a vessel that conveys lymph from the right upper quadrant of the
body into the bloodstream in the neck at the junction of the right internal jugular and the
right subclavian veins. About 1.25 cm long, the duct courses over the medial border of
the scalenus anterior. At its orifice are two semilunar valves that prevent venous blood
from flowing backward into the duct. Lymph drains into the right lymphatic duct from
numerous capillaries and vessels and from three lymphatic trunks in the right quadrant.
Formation and Flow of Lymph
Interstitial fluid that enters the lymphatic system through lymph capillaries in the
interstitial spaces is called lymph. This substance is filtered through lymph nodes rich in
white blood cells and then returned to the blood circulatory system through large lymph
ducts. Lymph maintains the fluid level in the body, fights infection, and by filtering
through the gastrointestinal tract, absorbs and transports fats.
In upright humans, up to 70% of the circulating blood volume is below the heart,
with the vast majority of this blood being stored in the venous capacitance vessels. As the
thin-walled veins readily distend with relatively small increases in transmural pressure,
compensatory mechanisms must be present to prevent the blood from pooling in the
extremities and aid in the return of blood to the heart. Two of the most widely accepted of
these mechanisms are the respiratory muscle pump and the skeletal muscle pump. The
respiratory muscle pump is the process by which pressure is changed during inhalation
and exhalation. This system drives lymph from the abdominal region to the thoracic
region during inhalation and the opposite way during exhalation. The skeletal muscle
pump is the process by which skeletal muscle contractions apply pressure to lymphatic
vessels and forces lymph toward the junction of the internal jugular and subclavian
veins.
Lymphatic Organs and Tissues
The lymphatic system is commonly divided into the primary lymphoid organs,
which are the sites of B and T cell maturation, and the secondary lymphoid organs, in
which further differentiation of lymphocytes occurs. Primary lymphoid organs include
the thymus, bone marrow, fetal liver, and, in birds, a structure called the bursa of
Fabricius. In humans the thymus and bone marrow are the key players in immune
function. All lymphocytes derive from stem cells in the bone marrow. Stem cells
destined to become B lymphocytes remain in the bone marrow as they mature, while
prospective T cells migrate to the thymus to undergo further growth. Mature B and T
lymphocytes exit the primary lymphoid organs and are transported via the bloodstream to
the secondary lymphoid organs, where they become activated by contact with foreign
materials, such as particulate matter and infectious agents, called antigens in this context.
Thymus
The thymus is a pyramid-shaped lymphoid organ that, in humans, is immediately
beneath the breastbone at the level of the heart. This organ is called thymus because its
shape resembles that of a thyme leaf.
The thymus is divided into two lobes, lying on either side of the midline of the
body, and into smaller divisions called lobules. It is covered by a dense connective tissue
capsule, which sends fibers into the body of the thymus for support. The thymus tissue is
distinguishable into an outer zone, the cortex, and an inner zone, the medulla.
The organ is composed principally of two types of cells, called, respectively,
lymphocytes and reticular cells. The reticular cells form a loose meshwork, as in a
lymph node, while the spaces between them are packed with lymphocytes. The cortex,
characterized by its heavy lymphocyte concentration, is the site of much lymphocytic
proliferation. Proliferation of lymphocytes in the thymus is distributed evenly throughout
the cortex, instead of germinal centers, as occurs in other lymphoid tissue. Some of the
daughter cells – called T (thymus derived) cells – that are produced in the cortex migrate
to the medulla, where they enter the bloodstream through the medullary veins, adding to
the lymphocytes seen in the peripheral blood and the lymphoid organs. Very few of these
T cells actually survive in the cortex because they are screened for possible autoimmune
functions and ultimately destroyed. The medulla also has thymic (Hassalls’s) corpuscles
which are concentric layers of five or six epithelial cells. Their role is currently unknown.
During the involution, or shrinking, of the thymus after puberty, the cortex
becomes thin. Lymphocytes disappear and are replaced by fat tissue from the partitions of
the lobules. The process of involution is never complete, and the bits of thymus tissue
that remain are probably sufficient to maintain its function.
The thymus differs structurally from other lymphoid organs in that is does not
have lymphatic vessels draining into it. It is not a filter like the lymph nodes, which are
situated so that microorganisms and other antigens are exposed to their cells. The thymic
lymphocytes are sealed from the rest of the body by a continuous layer of epithelial cells
that entirely surround the organ. While thus sequestered, the lymphocytes differentiate, or
acquire the capabilities to perform specialized tasks.
Lymph Nodes
Lymph nodes are any of the small, bean-shaped masses of lymphoid tissue
enclosed by a capsule of connective tissue that occur in association with the lymphatic
vessels. As part of the lymphatic system, lymph nodes serve as filters for the blood,
providing specialized tissues where foreign antigens can be trapped and exposed to cells
of the immune system for destruction. They are typically found concentrated near
junctions of the major lymphatic vessels, most prominently in the neck, groin, and
armpits.
Each lymph node is divided into two general regions, the capsule and the cortex.
The capsule is an outer layer of connective tissue. Underlying the capsule is the cortex, a
region containing mostly inactivated B and T lymphocytes plus numerous accessory cells
such as dendritic cells and macrophages. The cortex is further divided into two
functional areas: the outer cortex and inner cortex, or paracortex. These regions
surround an inner medulla, which consists primarily of activated antibodysecreting plasma cells.
Cells enter the lymph node through two primary routes. Lymph and its associated
cells enter through the afferent lymphatic vessels, which drain into each node through
its convex surface. These vessels may drain directly from th elymphatic capillaries, or
they may be connected to a previous node. Lymphocytes generally enter through
specialized blood vessels called high endothelial venules (HEVs). HEVs contain a single
layer of large endothelial cells that possess surface receptors specific for B and T
lymphocytes. As these cells pass through the HEVs, they bind to the receptors and are
carried into the paracortex of the lymph node.
The structural divisions within a lymph node serve different purposes. Most of the
lymphocytes within a node are “naive”—i.e., they have yet to encounter antigen—and
therefore must migrate to regions where they will be most effective in recognizing
foreign agents. B cells enter the paracortex through the HEVs and then migrate into the
outer cortex and join specialized dendritic cells and macrophages to form follicles.
Primary follicles consist of a resting B cellsurrounded by a loose network of dendritic
cells. After encountering a foreign antigen, the B cell becomes activated and is
surrounded by a more tightly packed association of dendritic cells and macrophages,
forming a germinal center. The germinal center in turn is enclosed by a mantle zone—a
ring of resting B cells and dendritic cells. The germinal center and mantle together
compose a secondary follicle, which is the site of antigen-dependent B-cell maturation.
The activated B cells then migrate through the paracortex to the medulla, where they
proliferate as antibody-secreting plasma cells. T cells enter the lymph node through the
HEVs and remain in the paracortex, where the cortical macrophages and dendritic cells
present antigenic peptides to the naive T cells, stimulating them to become
activated helper T cells or cytotoxic T lymphocytes. All activated lymphocytes migrate
through the medulla and enter the lymphatic circulation through the efferent lymphatic
vessel, which drains either into adjacent lymph nodes or ultimately into the thoracic duct,
a major vessel of the lymphatic system.
The central role played by lymph nodes in filtering microorganisms and other
undesired substances from the blood is critical to the functioning of the immune system
but also makes lymph nodes vulnerable to cancer. As cancerous cells spread by
metastasis, they can become trapped and concentrated in lymph nodes, where they
proliferate. Virtually all cancers have the potential of spreading to lymph nodes, a
condition that greatly complicates treatment. In most cases surgery alone will not remove
the cancer from the nodes, and therefore postoperative radiation or chemotherapy is
required.
Spleen
The spleen is an organ of the lymphatic system located in the left side of the
abdominal cavity under the diaphragm, the muscular partition between the abdomen and
the chest. In humans it is about the size of a fist and is well supplied with blood. As the
lymph nodes are filters for the lymphatic circulation, the spleen is the primary filtering
element for the blood. The organ also plays an important role in storing and releasing
certain types of immune cells that mediate tissue inflammation.
The spleen is encased in a thick connective-tissue capsule. Inside, the mass of
splenic tissue is of two types, the red pulp and the white pulp, which do not separate
into regions but intermingle and are distributed throughout the spleen. The white pulp
is lymphoid tissue that usually surrounds splenic blood vessels. The red pulp is a network
of channels (sinuses) filled with blood, and it is in the red pulp that most of the filtration
occurs.
The white pulp of the spleen contains such typical lymphoid elements
as plasma cells, lymphocytes, and lymphatic nodules, called follicles in the
spleen. Germinal centers in the white pulp serve as the sites of lymphocyte production.
Similar to the lymph nodes, the spleen reacts to microorganisms and other antigens that
reach the bloodstream by releasing special phagocytic cells. These cells reside in both red
and white pulp, and they serve to remove foreign material from the blood and to initiate
an immune reaction that results in the production of antibodies.
The channels of red pulp in the spleen serve as important reservoirs for large
quantities of phagocytic white blood cells (leukocytes) called monocytes. Studies have
shown that upon severe tissue injury, such as that sustained during a heart attack, the
spleen releases a legion of monocytes, which then travel through the bloodstream to the
site of injury. There they serve to regulate inflammation and to facilitate tissue healing. In
animals who have had their spleens removed, the monocyte response is not observed at
the site of tissue injury, and healing is less thorough. In addition, humans who have had
their spleens removed (a procedure known as a splenectomy) appear to be at increased
risk of cardiovascular disease as they age. It is suspected that the absence of immuneregulating factors released from the spleen is related to the increase in susceptibility to
cardiovascular disease in individuals who have undergone a splenectomy.
The red pulp has a specialized role in addition to filtration. It is the body’s major
site of the destruction of red blood cells (erythrocytes), which normally have a life span
of only 120 days. Degenerate red cells are removed from the circulation in the spleen,
and the hemoglobin that they contain is degraded to a readily excretable pigment and an
iron molecule that is recycled (i.e., used to produce new hemoglobin elsewhere).
Lymphatic Nodules
The lymph nodule is a small, localized collection of lymphoid tissue, usually
located in the loose connective tissue beneath wet epithelial membranes, as in
the digestive system, respiratory system, and urinary bladder. Lymph nodules form in
regions of frequent exposure to microorganisms or foreign materials and contribute to the
defense against them. The nodule differs from a lymph nodein that it is much smaller and
does not have a well-defined connective-tissue capsule as a boundary. It also does not
function as a filter, because it is not located along a lymphatic vessel. Lymph nodules
frequently contain germinal centers — sites for localized production of lymphocytes. In
the small intestine, collections of lymph nodules are called Peyer’s patches. The tonsils
are also local regions where the nodules have merged together.
The tonsil is a small mass of lymphatic tissue located in the wall of the pharynx at
the rear of the throat of man and other mammals. In man the term is used to designate any
of three sets of tonsils, most commonly the palatine tonsils. These are a pair of ovalshaped masses protruding from each side of the oral pharynx behind the mouth cavity.
The exposed surface of each tonsil is marked by numerous pits that lead to deeper
lymphatic tissue. Debris frequently lodges in the pits and causes inflammation, a
condition called tonsillitis. The function of the palatine tonsils is thought to be associated
with preventing infection in the respiratory and digestive tracts by producing antibodies
that help kill infective agents. Frequently, however, the tonsils themselves become the
objects of infection, and surgical removal (tonsillectomy) is required. Usually, children
are more prone to tonsillitis than adults, for the structures tend to degenerate and decrease
in size as one gets older.
Another major tonsillar pair is the pharyngeal tonsils, more commonly known
as adenoids. These are diffuse masses of lymphatic tissue located on the top wall of
the nasal pharynx. Enlargement of these tonsils may obstruct breathing through the
nose, interfere with sinus drainage, and lead to sinus and middle ear infections. When the
nasal respiratory passage is blocked, mouth breathing becomes necessary. Continued
mouth breathing puts stress on the developing facial bones in children and may cause
facial deformities. Surgical removal, often in conjunction with a tonsillectomy, is
frequently recommended in children. The adenoids tend to decrease in size during
adulthood.
The third pair of tonsils are the lingual tonsils, aggregations of lymphatic tissue
on the surface tissue at the base of the tongue. The surface of this tonsil has pits leading
to lower lymphatic tissue as in the other two tonsil types, but these pits are effectively
drained by small glands (mucous glands), and infection is rare.
Development of Lymphatic Tissues
The lymphatic vessels form, like the blood vessels, from hemangioblastic stem
cells which aggregate to form fine tubular vessels. The first signs of developing lymph
nodes are found already in the 5th week as so-called lymph sacs near where the inferior
cardinal vein and superior cardinal vein flow together and form the common cardinal
vein. Bilateral evaginations of the venous system above the later subclavian vein arise as
the jugular lymph sac and somewhat later below as the axillar lymph sac. In other
regions of the body further such evaginations arise: mesenteric, lumbar, iliac and
retroperitoneal sacs.
At the end of the embryonic period 3 bilateral systems have formed: the jugularaxillar lymphatic sac, the mesenterial lymphatic plexus, and the lumbo-inguinal
lymphatic plexus. They are connected through an exceptionally variable lymphatic
vessel system. The thoracic duct drains the lymph from the lower as well as the left
upper half of the body into the left venous angle between the jugular vein and the left
subclavian vein. The right arm and the right half of the head are drained by the jugular
and axillary lymphatic plexus, respectively, which empties into the right venous angle.
The lymph nodes develop in the early fetal period through a septation of the
lymph sacs by mesenchymal cells. The spaces thus delimited become the sinus of the
adult lymph nodes. Other mesenchymal cells build the connective tissue framework of
the lymph node and its capsule.
Aging and the Lymphatic System
After puberty, the thymus gland involutes and is replaced by connective tissue.
This means there is a decrease in the amount of thymosin produced, resulting in a decline
in the defense mechanisms of the body. As the immune system becomes less effective in
combating disease, the elderly become more prone to infections. This diminished
capacity can be as much as 50% of that of a younger person.
T-cells are less responsive to antigens; therefore, fewer cytotoxic T-cells respond
to an infection. This is partly because of the gradual decrease in size of the thymus gland
and its reduced production of the hormone, thymosin. Because the helper T-cell
production is reduced, B-cells become less responsive, and antibody levels do not rise as
quickly after antigen exposure. Depressed lymphocyte function is also accompanied by a
decrease in macrophage activity. The result is an increased susceptibility to viral and
bacterial illnesses.