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Lecture 17
The Lymphatic System and Immunity
Lymph and Lymphatic Vessels
 Functions of the Lymphatic System
o __________________ recovery
 Fluid filters from our blood capillaries into tissue spaces
 Only 85% of that is reabsorbed by the capillaries, leaving behind
about 2 to 4 L of ______________ and up to 50% of the plasma
protein per day
 The lymphatic system reabsorbs this excess and returns it to the
o Immunity
 As the lymphatic system recovers excess tissue fluid, it also picks
up foreign cells and ____________________ from the tissues
 Some of these are pathogens, agents that have the potential to
cause disease
 On its way back to the bloodstream, the fluid passes through lymph
nodes, where immune cells stand guard against pathogens and
______________ protective immune responses
o Lipid Absorption
 In the small intestine, special lymphatic vessels called
______________ absorb dietary lipids that cannot be absorbed by
intestinal blood capillaries
 Lymph
o Lymph is usually a clear, colorless fluid, similar to blood plasma but low
in protein
o Its composition varies substantially from place to place
 After a meal, lymph draining from the small intestine has a milky
appearance because of its high ______________ content
 Intestinal lymph is called chile
o Lymph leaving the lymph nodes contains a large number of lymphocytes
o Lymph can also contain macrophages, hormones, bacteria, viruses,
cellular debris, and even traveling ______________ cells
 Lymphatic Vessels
o Lymph flows through a system of lymphatic vessels similar to blood
o These begin with microscopic lymphatic capillaries (terminal lymphatics)
 Lymphatic capillaries penetrate nearly _______________ tissue of
the body except for the CNS, cartilage, cornea, bone, and bone
 They are closely associated with blood capillaries, but are closed at
one end
The cells of the capillaries do not have tight junctions, and the gaps
are so large that they bacteria, _____________________, and other
cells and particles can enter along with tissue fluid
o Larger lymphatic vessels are similar to veins in their histology
 They have a tunica interna with and endothelium and valves,
 They have a tunica _______________ with elastic fibers and
smooth muscle
 They have a thin outer tunica externa
o Lymphatic vessels converge along their path and become larger and larger
o Lymphatic capillaries converge to form collecting vessels
 These travel alongside veins and arteries
o Collecting vessels alternate with lymph nodes
 Lymph trickles through the node, where bacteria are phagocitized
and immune cells __________________ the fluid for foreign
o Collecting vessels combine to form lymphatic trunks which drain major
portions of the body
o Lymphatic trunks converge to form two collecting ducts
 The right lymphatic duct receives lymphatic drainage from the
right arm and right side of the thorax and head, and empties into
the right ____________________ vein
 The thoracic duct on the left, is larger and longer and drains all of
the body below the diaphragm and the left arm and left side of the
head and thorax
The Flow of Lymph
o Lymph flows at ____________________ pressure and speed than venous
o The primary mechanism of flow is rhythmic contractions of the lymphatic
vessels, which contract when the flowing lymph stretches them
o Valves of lymphatic vessels prevent fluid from flowing backward
o Lymph flow is also produced by skeletal muscles squeezing the lymphatic
o Where the collecting ducts empty into the subclavian veins, the rapidly
flowing _____________________ draws the lymph into it
Lymphatic Cells, Tissues, and Organs
 Lymphatic Cells and Modes of Defense
o Principle cell types include:
 Natural killer cells
 These are large lymphocytes that provide a nonspecific
defense against __________________, viruses, tissue
transplants, and cancer.
 Their constant patrolling of the body for defective cells is
called immunological surveillance
 T lymphocytes
These are so-named because they develop for a time in the
 Cytotoxic T cells are the only T lymphocytes that directly
attack and kill other cells
 Helper T cells help other immune cells respond to the
 Suppressor T cells ___________________ the immune
 Memory T cells provide long-lesting memory of an
antigen, so that upon re-exposure, the immune system can
neutralize the antigen quickly
 B lymphocytes
 These cells mature in bone marrow
 When activated, they differentiate into plasma cells, which
secrete circulating _______________
 Macrophages
 These cells develop from monocytes that have emigrated
from the bloodstream
 They are large, _____________________ cells that ingest
and destroy tissue debris, dead neutrophils, bacteria, and
other foreign matter
 They can act as antigen-presenting cells (APCs) to activate
immune response
 Dendritic cells
 These are branched macrophages found in the epidermis,
mucous membranes, and lymphatic organs
 They engulf foreign matter by receptor–mediated
_______________, but otherwise function like
 Reticular cells
 These are branched stationary cells that contribute to the
connective framework of the lymphatic organs
 They act as APCs
Lymphatic Tissues
o Lymphatic tissues are aggregations of lymphocytes in the connective
tissues of mucous membranes and various organs
o Diffuse lymphatic tissues is the simplest form
 Lymphocytes are scattered rather than densely clustered
 It is prevalent in body passages that are open to the
 Respiratory, digestive, urinary, and reproductive tracts
o Lymphatic nodules are areas where lymphocytes are congregated in dense
 These may come and go as _________________ invade the tissues
and the immune system answers the challenge
Lymphatic nodules are relatively constant features of the lymph
nodes, tonsils, and appendix
Red Bone Marrow
o Red bone marrow is involved in ____________________ and immunity
o It is soft, loosely organized, highly vascular material within the endosteum
o It produces all classes of formed elements in the blood
o Within the bone marrow are channels called _________________ that
drain into a longitudinal vein that exits the bone
o The sinusoids are lined by endothelial cells and are surrounded by
reticular cells and reticular fibers
o The reticular cells secrete colony-stimulating factors that induce the
formation of various _________________ types
o Spaces between the sinusoids are composed of macrophages and blood
cells in all stages of development
o As they mature, they push their way through the reticular and endothelial
cells to enter the sinus and flow away in the bloodstream
The Thymus
o The thymus is divided into several angular _______________
o Each lobule has a cortex and a medulla, which contains T lymphocytes
o Reticular epithelial cells seal off the cortex from the medulla and isolate
developing lymphocytes from blood-borne antigens
 These cells also secrete thymic __________________ that regulate
T cell development and activity
Lymph Nodes
o These are the most numerous lymphatic organs, numbering in the
o They cleanse the lymph, and they alert the immune system to pathogens
o Each node is an elongated or ____________-shaped structure, less than 3
cm long
o They have several functions:
 monitor the lymph for foreign antigens
 remove impurities before it returns to the bloodstream
 contribute ________________________ to the lymph and blood
 mount immune responses to foreign antigens
o These are patches of lymphatic tissue located at the entrance to the
o They guard against ingested and _______________ pathogens
o Each is covered by an epithelium and has deep pits called tonsilar crypts
lined by lymphatic nodules
 The crypts often contain food debris, dead leukocytes, and bacteria
o There are three main sets of tonsils
 Pharyngeal tonsils (adenoids) are on the wall of the pharynx, just
behind the nasal cavity
 Palatine tonsils are at the posterior margin of the ______________
Lingual tonsils are concentrated in a patch on each side of the root
of the tongue
The Spleen
o The largest lymphatic organ is the spleen
 It’s up to 10 cm long and weighs up to 160 g
 It is located in the upper left part of the abdomen, just inferior to
the ___________________ and dorsolateral to the stomach
 It has two tissue types:
 Red pulp
o Sinuses gorged with concentrated erythrocytes
 White pulp
o Consists of lymphocytes and macrophages
aggregated like sleeves along small branches of the
______________ artery
 It produces blood cells in the fetus and may resume this
role if necessary in adults
 Lymphocytes and macrophages of the white pulp
_____________ the blood for foreign antigens, much like
the lymph nodes do the lymph
 The spleen is an “erythrocyte _________________”
o Old, fragile RBCs rupture as they squeeze through
the capillary walls into the sinuses
o Macrophages phagocityze their remains, just as they
dispose of blood-borne bacteria and other cellular
 The spleen also helps to stabilize blood volume by
transferring excess plasma from the bloodstream into the
lymphatic system
The Lymphatic System in Relation to Immunity
 The immune system is not an ____________ system; it’s a population of diseasefighting cells that are found in many areas of the body
 There are two forms of specific immunity called humoral immunity and cellular
o Humoral immunity is carried out by B lymphocytes and antibodies
 It is called humoral because the antibodies freely circulate in the
body fluids (________________ are body fluids)
o Cellular immunity is carried out by cytotoxic T cells
 B Cells and T cells have some things in common:
o Both begin development with pluipotent stem cells in the red bone marrow
o PPSCs divide and give rise to lymphocyte colony-forming units, which
ultimately produce B and T lymphocytes
o Before they can take part in immune reactions, both types must develop
antigen receptors on their surface
 This gives them ________________________, the ability to
recognize, bind, and respond to an antigen
o The body must get rid of lymphocytes that react against its own antigens
so that the immune system will not attack our own organs
 The destruction or deactivation of self-reactive lymphocytes is
called ____________________ selection
B Cells and Humoral Immunity
o B cells achieve immunocompetence and go through negative selection in
red bone marrow
o Many of the mature immunocompetent B cells remain there, while many
more disperse and populate other sites such as the mucous membranes,
spleen, and the cortical nodules of the lymph nodes
o When one of these B cells encounters an antigen, it internalizes and
__________________ it, and presents fragments of the antigen to a helper
T cell
o The helper T cell secretes chemical helper factors that stimulate the B cell
to divide still more
o Most of its daughter cells differentiate into plasma cells
 Plasma cells secrete antibodies
 The antibodies travel throughout the body in the blood and other
o Some B cells, instead of becoming plasma cells, become
_______________ cells
 They may live for years, and respond very quickly if they ever
encounter the same antigen again.
T Cells and Cellular Immunity
o T lymphocytes leave the bone marrow before reaching maturity
o They migrate to the cortex of the thymus where they achieve
immunocompetence and undergo selection
o Mature T cells then move to the thymic medulla, where they may leave the
thymus via the blood and lymphatic vessels
o The T cells then _________________ the various lymphatic organs
o When cytotoxic cells encounter an enemy cell, they attack it directly and
destroy it by a lethal hit of toxic chemicals
 This is why immunity carried out by T cells is called cellular