Download Chapter 21 - Dr. Gerry Cronin

Chapter 22
The Lymphatic System and
Immunity
The Lymphatic System
• A system consisting of lymphatic vessels through
which a clear fluid (lymph) passes
• The major functions of the lymphatic system
include:
• Draining interstitial fluid
• Transporting dietary lipids absorbed by the
gastrointestinal tract to the blood
• Facilitating the immune responses
The Lymphatic System
• Components of the lymphatic
system include:
• Lymphatic capillaries
• Lymphatic vessels
• Lymph nodes
• Lymphatic trunks
• Lymphatic ducts
• Primary lymphatic organs
• Secondary lymphatic organs
and tissues
Lymphatic Vessels and Fluid
• Lymph is a clear to milky fluid in the extracellular
fluid compartment. Extracellular fluids include:
• Plasma – the liquid component of blood
• Interstitial fluid – the clear fluid filtered through capillary
walls when it enters the “interstitium” (space between
cells, also called the intracellular space)
• Lymphatic fluid – the unaltered interstitial fluid that
enters the lymphatic vessels. In the GI tract, lymphatic
fluids also include absorbed dietary lipids.
Lymphatic Vessels and Fluid
• The flow of lymph fluid is always from the
periphery towards the central vasculature.
• It starts as interstitial fluid.
• Then enters lymphatic
capillaries.
• It travels in lymphatic
vessels to the regional
lymph nodes…
Lymphatic Vessels and Fluid
• The flow of lymph fluid continued…
• Lymph ascends or descends to the thorax, either to
the Left or Right Lymphatic Duct.
• Lymph fluid’s final destination is the bloodstream, as
it enters through the Subclavian veins.
Lymphatic Vessels and Fluid
• Lymphatic capillaries are slightly larger than
blood capillaries and have a unique one-way
structure.
• The ends of endothelial cells overlap and permit
interstitial fluid to flow in, but not out.
• Anchoring filaments pull openings wider when
interstitial fluid accumulates.
• There are specialized lymphatic capillaries called
lacteals that take up dietary lipids in the small
intestine.
• Chyle is the name of this “lymph with lipids”.
Lymphatic Vessels and Fluid
Lymphatic capillaries showing blind ends and one way flow
Lymphatic Vessels and Fluid
• Lymphatic capillaries unite to form larger
lymphatic vessels which resemble veins in
structure but have thinner walls and more
valves.
• Lymphatic vessels pass
through lymph nodes –
encapsulated organs with
masses of B and T cells.
• Function as lymph filters
Lymphatic Vessels and Fluid
Lymphatic Vessels and Fluid
• Lymphatic fluid is moved by pressure in the
interstitial space and the milking action of skeletal
muscle contractions and respiratory movements.
• An obstruction or
malfunction of lymph
flow leads to edema
from fluid
accumulation in
interstitial spaces.
Lymphatic Organs
• The lymphatic system is composed
of a number of primary and
secondary organs and tissues
widely distributed throughout
the body - all with the purpose
of facilitating the immune
response.
Lymphatic Organs
• Primary lymph organs are the bone marrow and
thymus.
– Sites where stem cells divide and become
immunocompetent (capable of
mounting an immune response)
• Secondary lymphatic organs are
sites where most immune responses
occur, including the spleen and
lymph nodes, and other lymphoid
tissues such as the tonsils.
Lymphatic Organs
• Thymus
• The outer cortex is composed of a large number of
immature T cells which migrate from their birthplace in red bone marrow .
• They proliferate and begin to mature with the help of
Dendritic cells (derived from monocytes) and specialized
epithelial cells (help educate T cells through positive
selection) – only about 25% survive.
• The inner medulla is composed of more mature T
cells.
Lymphatic Organs
• The thymus slightly protrudes from the
mediastinum into the lower neck.
• It is a palpable 70g
in infants, atrophies
by puberty, and is
scarcely distinguishable
from surrounding fatty
tissue by old age.
Lymphatic Organs
• There are about 600 lymph nodes scattered along
lymphatic vessels (in superficial and deep groups)
that serve as filters to trap and destroy
foreign objects in lymph fluid.
• Important group of regional
lymph nodes include:
– Submandibular
– Cervical
– Axillary
• Mediastinal
• Inguinal
Lymphatic Organs
• Lymph fluid enters the node through afferent
vessels and is directed towards the central
medullary sinuses.
• Efferent vessels convey
lymph, antibodies and
activated T cells out of
the node at an indentation
called the hilum.
Lymphatic Organs
• The spleen is the body’s largest mass of
lymphatic tissue.
• The parenchyma of the organ consists of:
• White pulp - lymphatic tissue where lymphocytes
and macrophages carry out immune function
• Red pulp – blood-filled venous sinuses where
platelets are stored and
old red cells
are destroyed
Lymphatic Organs
The Immune Response
• Our immune response includes innate and
adaptive responses:
Innate Immunity
• The innate immune response is present at birth.
It is non-specific and non-adaptive.
• It includes our first
line of external,
physical, and
chemical barriers
provided by the
skin and mucous
membranes.
Innate Immunity
• Our nonspecific innate
immune response also
includes various
internal defenses such
as antimicrobial
substances, natural
killer cells, phagocytes,
inflammation, and
fever.
Innate Immunity
• Internal defenses:
– Phagocytes
• Wandering and
fixed macrophages
– Natural killer (NK) cells
– Endogenous antimicrobials
– Complement system
– Iron-binding proteins
– Interferon
Innate Immunity
• Phagocytosis is a non-specific process wherein
neutrophils and macrophages (from monocytes)
migrate to an infected area. There are 5 steps:
•
•
•
•
•
Chemotaxis
Adherence
Ingestion
Digestion
Killing
Innate Immunity
Innate Immunity
• Fever is an abnormally high body temperature
due to resetting of the hypothalamic thermostat.
• Non-specific response:
• speeds up body reactions
• increases the effects of endogenous antimicrobials
• sequesters nutrients from microbes
Innate Immunity
• Inflammation is defensive response of almost all
body tissues to damage of any kind (infection,
burns, cuts, etc.).
• The four characteristic signs and symptoms of
inflammation are redness, pain, heat, and swelling.
• It is a non-specific attempt to dispose of microbes
and foreign materials, dilute toxins, and prepare for
healing.
Innate Immunity
• The inflammatory response has three basic
stages:
• Vasodilation and increased permeability
• Emigration (movement) of
phagocytes from the
blood into the
interstitial space
and then to site
of damage
• Tissue repair
Innate Immunity
Vasodilation allows more blood to flow to the
damaged area which helps remove toxins and debris.
• Increased permeability permits entrance of defensive
proteins (antibodies and clotting factors) to site of
injury

• Other inflammatory mediators include histamine, kinins,
prostaglandins (PGs), leukotrienes (LTs), and complement.
Innate Immunity
• Emigration of phagocytes depends on
chemotaxis
• Neutrophils predominate in early stages but die off
quickly.
• Monocytes transform into macrophages and become
more potent phagocytes than neutrophils.
• Pus is a mass of dead phagocytes and damaged tissue.
• Pus formation occurs in most inflammatory responses and
usually continues until the infection subside.