Lymphatic System and
Immune System
2.
Part 1
1. Function and Components of
Lymphatic System
Innate Defenses of Immune System
Objectives
1.
2.
3.
4.
5.
6.
Primary functions of lymphatic system
Unique structure of lymphatic vessels and
organs
Connection of lymphatic and immune
systems
Two components of immune system
Structures and cells of the innate division of
immune system
Steps of inflammatory response
2 semi-independent parts of Lymphatic
System

Lymphatic vessels
• Meandering network of vessels and nodes
• Transport back to blood any fluids that have
escaped from the blood vascular system

Lymphoid tissues, nodes and organs
• Scattered throughout body
• House phagocytic cells and lymphocytes…
ready for body defense and resistance to
disease
Lymphatic Vessels





3 L per day of fluid seeps out
of capillaries to become part
of “interstitial fluid”…this
would cause edema if not
returned to blood vascular
system
Lymphatic vessels pick up
this “lymph”
Minivalve on lymph vessels
Detours to lymph nodes to
clean lymph of debris
Returned to veins through
large lymph ducts
Lymph




Leaked fluid from blood vessels that has
become interstitial fluid and now must be
returned to blood
Contains plasma proteins
Lymph means “clear water”
About 3L per day
Lymphatic Vessels


One-way system, lymph flows toward the
heart
Lymph vessels (lymphatics) include:
• Lymphatic capillaries
• Lymphatic collecting vessels
• Lymphatic trunks and ducts
Venous system
Arterial system
Heart
Lymphatic
system:
Lymph duct
Lymph trunk
Lymph node
Lymphatic
collecting vessels,
with valves
Blood
capillaries
Lymphatic
capillary
Tissue
fluid
Tissue cell Blood
Lymphatic
capillaries
capillaries
(a) Structural relationship between a
capillary bed of the blood vascular
system and lymphatic capillaries.
Filaments anchored
to connective tissue
Endothelial cell
Flaplike minivalve
Fibroblast in loose
connective tissue
(b) Lymphatic capillaries are blind-ended tubes in which
adjacent endothelial cells overlap each other,
forming flaplike minivalves.
Figure 19.1
Lymphatic Capillaries

Similar to blood capillaries, except
• Very permeable (take up cell debris,
•
pathogens, and cancer cells)
Endothelial cells overlap to form one-way
minivalves, and are anchored by collagen
filaments, preventing collapse of capillaries
Lymphatic Capillaries


Absent from bones, teeth, bone marrow
and the CNS
Lacteals: specialized lymph capillaries
present in intestinal mucosa
• Absorb digested fat and deliver fatty lymph
(chyle) to the blood
Filaments anchored
to connective tissue
Endothelial cell
Flaplike minivalve
Fibroblast in loose
connective tissue
(b) Lymphatic capillaries are blind-ended tubes in which
adjacent endothelial cells overlap each other,
forming flaplike minivalves.
Figure 19.1b
Lymphatic Collecting Vessels

Similar to veins, except

Collecting vessels in the skin travel with
superficial veins
Deep vessels travel with arteries
Nutrients are supplied from branching
vasa vasorum


• Have thinner walls, with more internal valves
• Anastomose more frequently
Lymphatic Trunks

Formed by the union of the largest
collecting ducts
• Paired lumbar
• Paired bronchomediastinal
• Paired subclavian
• Paired jugular trunks
• A single intestinal trunk
Lymphatic Ducts

Lymph is delivered into one of two large ducts
• Right lymphatic duct drains the right upper arm and
•

the right side of the head and thorax
Thoracic duct arises from the cisterna chyli and
drains the rest of the body
Each empties lymph into venous circulation at
the junction of the internal jugular and
subclavian veins on its own side of the body
Flow of Lymph
Lymphatic Capillaries
Lymphatic Collecting Vessels
Lymphatic Trunks
Lymphatic Ducts
Terms to remember:
Lacteals
Right Lymphatic Duct/ Thoracic Duct
Cisterna chyli
Regional
lymph nodes:
Cervical nodes
Internal jugular vein
Entrance of right
lymphatic duct into vein
Entrance of thoracic
duct into vein
Axillary nodes
Thoracic duct
Cisterna chyli
Lymphatic
collecting vessels
Aorta
Inguinal nodes
Drained by the right
lymphatic duct
Drained by the
thoracic duct
(a) General distribution of lymphatic collecting vessels
and regional lymph nodes.
Figure 19.2a
Right jugular trunk
Right lymphatic duct
Right subclavian trunk
Right subclavian vein
Right bronchomediastinal trunk
Brachiocephalic veins
Superior vena cava
Azygos vein
Internal jugular veins
Esophagus
Trachea
Left subclavian trunk
Left jugular trunk
Left subclavian vein
Entrance of thoracic
duct into vein
Left bronchomediastinal
trunk
Ribs
Thoracic duct
Hemiazygos vein
Cisterna chyli
Right lumbar trunk
Left lumbar trunk
Inferior vena cava
Intestinal trunk
(b) Major lymphatic trunks and ducts in relation to veins and
surrounding structures. Anterior view of thoracic and abdominal wall.
Figure 19.2b
Lymph Transport

Lymph is propelled by
• Contractions of smooth muscle in the walls of
•
•
•
•
the lymphatic trunks
Lymph vessels bundled together in
connective tissue sheaths with blood vessels,
so pulsations of nearby arteries propel lymph
Milking action of active skeletal muscles so
flows faster when active
Pressure changes in thorax during breathing
Valves prevent backflow
Lymphoid Cells

Lymphocytes the main warriors of the immune
system
Two main varieties

T cells and B cells protect against antigens

• T cells (T lymphocytes)
• B cells (B lymphocytes)
• Anything the body perceives as foreign
• Bacteria and their toxins; viruses
• Mismatched RBCs or cancer cells
Lymphocytes

T cells

B cells
• Manage the immune response
• Attack and destroy foreign cells
• Produce plasma cells, which secrete
antibodies
Other Lymphoid Cells



Macrophages phagocytize foreign
substances and help activate T cells
Dendritic cells capture antigens and
deliver them to lymph nodes
Reticular cells produce stroma that
supports other cells in lymphoid organs
• Fibroblast-like cells
Cell types
Macrophage
Extracellular
matrix
Ground substance
Fibers
• Collagen fiber
• Elastic fiber
• Reticular fiber
Fibroblast
Lymphocyte
Fat cell
Capillary
Mast cell
Neutrophil
Figure 4.7
Macrophage
Reticular cells on
reticular fibers
Lymphocytes
Medullary sinus
Reticular fiber
Figure 19.3
Lymphoid Tissue



Houses and provides a proliferation site
for lymphocytes
Furnishes a surveillance vantage point
Two main types
• Diffuse lymphatic tissue
• Lymphatic follicles
Lymphoid Tissue

Diffuse lymphatic tissue comprises
scattered reticular tissue elements in
every body organ
• Larger collections in the lamina propria of
mucous membranes and lymphoid organs
Lymphoid Tissue

Lymphatic follicles (nodules) are solid,
spherical bodies of tightly packed
reticular elements and cells
• Germinal center composed of dendritic and B
•
cells
May form part of larger lymphoid organs
Lymph Nodes



Principal lymphoid organs of the body
Embedded in connective tissue, in
clusters along lymphatic vessels
Near the body surface in inguinal,
axillary, and cervical regions of the body
Regional
lymph nodes:
Cervical nodes
Internal jugular vein
Entrance of right
lymphatic duct into vein
Entrance of thoracic
duct into vein
Axillary nodes
Thoracic duct
Cisterna chyli
Lymphatic
collecting vessels
Aorta
Inguinal nodes
Drained by the right
lymphatic duct
Drained by the
thoracic duct
(a) General distribution of lymphatic collecting vessels
and regional lymph nodes.
Figure 19.2a
Lymph Nodes

Functions
1. Filter lymph—macrophages destroy
2.
microorganisms and debris
Immune system—lymphocytes are activated
and mount an attack against antigens
Structure of a Lymph Node




Bean shaped
External fibrous capsule
Trabeculae extend inward and divide the
node into compartments
Two histologically distinct regions
• Cortex
• Medulla
Structure of a Lymph Node




Cortex contains follicles with germinal
centers, heavy with dividing B cells
Dendritic cells nearly encapsulate the
follicles
Deep cortex houses T cells in transit
T cells circulate continuously among the
blood, lymph nodes, and lymphatic
stream
Afferent lymphatic
vessels
Cortex
• Lymphoid follicle
• Germinal center
• Subcapsular sinus
Efferent lymphatic
vessels
Hilum
Medulla:
• Medullary cord
• Medullary sinus
Trabeculae
Capsule
(a) Longitudinal view of the internal structure of a lymph
node and associated lymphatics
Figure 19.4a
Structure of a Lymph Node


Medullary cords extend inward from the
cortex and contain B cells, T cells, and
plasma cells
Lymph sinuses contain macrophages
Follicles
Trabecula
Subcapsular sinus
Capsule
Medullary cords
Medullary sinuses
(b) Photomicrograph of part of a lymph node (72x)
Figure 19.4b
Circulation in the Lymph Nodes

Lymph
• Enters via afferent lymphatic vessels
• Travels through large subcapsular sinus and
•

smaller sinuses
Exits the node at the hilus via efferent vessels
Fewer efferent vessels, causing flow of
lymph to stagnate, allowing lymphocytes
and macrophages time to carry out
functions
Cortex
• Lymphoid follicle
Afferent
lymphatic
vessels
• Germinal center
• Subcapsular sinus
Efferent
lymphatic Follicles
vessels
Trabecula
Subcapsular
sinus
Hilum
Medulla:
Capsule
• Medullary
cord
• Medullary
sinus
Medullary
cords
Trabeculae
Capsule
(a) Longitudinal view of the internal structure
of a lymph node and associated lymphatics
Medullary
sinuses
(b) Photomicrograph of part of a
lymph node (72x)
Figure 19.4
Homeostatic imbalance of
Lymph Nodes

Swollen Lymph Nodes
• Trap large numbers of bacteria/ inflammed
• Infected lymph nodes= buboes
• Bubonic plague

Cancer Sites
• Secondary cancer sites as cancer cells
•
metastasize
Cancer infiltrated lymph nodes are swollen but
not painful
Spleen



Largest lymphoid organ
Served by splenic artery and vein, which
enter and exit at the hilus
Functions
• Site of lymphocyte proliferation and immune
•
surveillance and response
Cleanses the blood of aged cells and platelets
and debris
Diaphragm
Spleen
Adrenal
gland
Left
kidney
Splenic
artery
Pancreas
(c) Photograph of the spleen in its normal position
in the abdominal cavity, anterior view.
Figure 19.6c
Spleen





Stores breakdown products of RBCs
(e.g., iron) for later reuse
Stores blood platelets
Site of fetal erythrocyte production
(normally ceases after birth)
Has a fibrous capsule and trabeculae
Contains lymphocytes, macrophages,
and huge numbers of erythrocytes
Structure of the Spleen

Two distinct areas
• White pulp around central arteries
• Mostly lymphocytes on reticular fibers and
•
involved in immune functions
Red pulp in venous sinuses and splenic cords
• Rich in macrophages for disposal of worn-out
RBCs and bloodborne pathogens
Capsule
Trabecula
Splenic cords
Venous sinuses
Arterioles and
capillaries
Splenic
artery
Splenic
vein Hilum
Red pulp
White pulp
Central artery
(a) Diagram of the
spleen, anterior view
Splenic artery
Splenic vein
(b) Diagram of spleen histology
Figure 19.6a,b
Thymus

Size with age
• In infants, it is found in the inferior neck and
•
•
extends into the mediastinum, where it
partially overlies the heart
Increases in size and is most active during
childhood
Stops growing during adolescence and then
gradually atrophies
Thymus



Thymic lobes contain an outer cortex
and inner medulla
Cortex contains densely packed
lymphocytes and scattered
macrophages
Medulla contains fewer lymphocytes and
thymic (Hassall’s) corpuscles involved in
regulatory T cell development
Thymic (Hassall’s)
corpuscle
Medulla
Cortex
Figure 19.7
Thymus

Differs from other lymphoid organs in
important ways
• It functions strictly in T lymphocyte maturation
• It does not directly fight antigens


The stroma of the thymus consists of starshaped epithelial cells (not reticular fibers)
These thymocytes provide the environment
in which T lymphocytes become
immunocompetent
Tonsils


Simplest lymphoid organs
Form a ring of lymphatic tissue around the pharynx
• Palatine tonsils—at posterior end of the oral cavity
• Lingual tonsils—grouped at the base of the tongue
• Pharyngeal tonsil—in posterior wall of the nasopharynx
• Tubal tonsils—surrounding the openings of the
auditory tubes into the pharynx
• Pharyngotympanic tube
Tonsils




Contain follicles with germinal centers
Are not fully encapsulated
Epithelial tissue overlying tonsil masses
invaginates, forming tonsillar crypts
Crypts trap and destroy bacteria and
particulate matter
Pharyngeal tonsil
Palatine tonsil
Lingual tonsil
Tonsil
Tonsillar crypt
Germinal centers
in lymphoid follicles
Figure 19.8
Aggregates of Lymphoid
Follicles

Peyer’s patches
• Clusters of lymphoid follicles
• In the wall of the distal portion of the small
•

intestine
Similar structures are also found in the appendix
Peyer’s patches and the appendix
• Destroy bacteria, preventing them from breaching
•
the intestinal wall
Generate “memory” lymphocytes
Lymphoid nodules
(follicles) of
Peyer’s patch
Smooth muscle in
the intestinal wall
Figure 19.9
MALT: a Collection of Lymph tissues

Mucosa-associated lymphatic tissue,
including
• Peyer’s patches, tonsils, and the appendix
•

(digestive tract)
Lymphoid nodules in the walls of the bronchi
(respiratory tract)
Protects the digestive and respiratory
systems from foreign matter
Quick review of Lymphatics
1.
2.
3.
4.
5.
Why do lymph nodes or
tonsils swell?
Which cells produce T
cells?...antibodies?
What is lymph node
connection with cancer?
What is lymph? How
much transported?
What lymph vessel
empties into blood
system? Where?
Name the organ:
6. “youth organ” / doesn’t
actively clean lymph
7. Destroys worn out RBC
and returns to liver
8. Filters lymph/ has more
afferent vessels than
efferent
9. Stores platelets/ a blood
reservoir
10. Simplest lymph organ/
helps immune cells
create memory
Immunity



Resistance to disease
Functional system rather than organ
system
Immune system has two intrinsic
systems
• Innate (nonspecific) defense system
• Adaptive (specific) defense system
Immunity
1.
Innate defense system has two lines of
defense
•
•
First line of defense is external body
membranes (skin and mucosae)
Second line of defense is antimicrobial
proteins, phagocytes, and other cells
•
•
Inhibit spread of invaders
Inflammation is its most important mechanism
Immunity
2.
Adaptive defense system
• Third line of defense attacks particular
foreign substances
• Takes longer to react than the innate system

Innate and adaptive defenses are
deeply intertwined
Surface barriers
• Skin
• Mucous membranes
Innate
defenses
Internal defenses
• Phagocytes
• NK cells
• Inflammation
• Antimicrobial proteins
• Fever
Humoral immunity
• B cells
Adaptive
defenses
Cellular immunity
• T cells
Figure 20.1
Innate Defenses

Surface barriers
• Skin, mucous membranes, and their
secretions
• Physical barrier to most microorganisms
• Keratin is resistant to weak acids and bases,
bacterial enzymes, and toxins
• Mucosae provide similar mechanical barriers
Surface Barriers

Protective chemicals inhibit or destroy
microorganisms
• Skin acidity – pH 3-5
• Lipids in sebum and dermcidin in sweat
• HCl and protein-digesting enzymes of
•
•
stomach mucosae
Lysozyme of saliva and lacrimal fluid
Mucus
Surface Barriers

Respiratory system modifications
• Mucus-coated hairs in the nose
• Cilia of upper respiratory tract sweep dustand bacteria-laden mucus from lower
respiratory passages
Internal Defenses: Cells and
Chemicals

Necessary if microorganisms invade
deeper tissues
• Phagocytes
• Natural killer (NK) cells
• Inflammatory response (macrophages, mast
•
•
cells, WBCs, and inflammatory chemicals)
Antimicrobial proteins (interferons and
complement proteins)
Fever
Phagocytes: Macrophages

Macrophages develop from monocytes
to become the chief phagocytic cells
• Free macrophages wander through tissue spaces
• E.g., alveolar macrophages
• Fixed macrophages are permanent residents of
some organs
• E.g., Kupffer cells (liver) and microglia (brain)
Phagocytes: Neutrophils

Neutrophils
• Become phagocytic on encountering
•
infectious material in tissues
Most abundant white blood cell
Mechanism of Phagocytosis
Step 1: Adherence of phagocyte to
pathogen
• Facilitated by opsonization—coating of
pathogen by complement proteins or
antibodies
Innate defenses
Internal defenses
(a) A macrophage (purple) uses its cytoplasmic
extensions to pull spherical bacteria (green)
toward it. Scanning electron micrograph (1750x).
Figure 20.2a
1 Phagocyte
adheres to
pathogens or debris.
Lysosome
Phagosome
(phagocytic
vesicle)
Acid
hydrolase
enzymes
(b) Events of phagocytosis.
2 Phagocyte forms
pseudopods that
eventually engulf the
particles forming a
phagosome.
3 Lysosome fuses
with the phagocytic
vesicle, forming a
phagolysosome.
4 Lysosomal
enzymes digest the
particles, leaving a
residual body.
5 Exocytosis of the
vesicle removes
indigestible and
residual material.
Figure 20.2b
Mechanism of Phagocytosis

Destruction of pathogens
• Acidification and digestion by lysosomal
•
enzymes
Respiratory burst
• Release of cell-killing free radicals
• Activation of additional enzymes
• Oxidizing chemicals (e.g. H2O2)
• Defensins (in neutrophils)
Natural Killer (NK) Cells





Large granular lymphocytes
Target cells that lack “self” cell-surface
receptors
Induce apoptosis in cancer cells and virusinfected cells
Secrete potent chemicals that enhance the
inflammatory response
Perforins/ granzymes
Inflammatory Response




Triggered whenever body tissues are
injured or infected
Prevents the spread of damaging agents
Disposes of cell debris and pathogens
Sets the stage for repair
Inflammatory Response

Cardinal signs of acute inflammation:
1. Redness
2. Heat
3. Swelling
4. Pain
(And sometimes 5. Impairment of function)
Inflammatory Response



Macrophages and epithelial cells of
boundary tissues bear Toll-like receptors
(TLRs)
TLRs recognize specific classes of infecting
microbes
Activated TLRs trigger the release of
cytokines that promote inflammation and
attract WBCs to area
Inflammatory Response

Inflammatory mediators
• Histamine (from mast cells)
• Blood proteins
• Kinins, prostaglandins (PGs), leukotrienes,
and complement
•Released by injured tissue, phagocytes,
lymphocytes, basophils, and mast cells
Vasodilation and Increased
Vascular Permeability


Inflammatory chemicals cause
• Dilation of arterioles, resulting in
hyperemia
• Increased permeability of local
capillaries and edema (leakage of
exudate)
Exudate contains proteins, clotting
factors, and antibodies
Inflammatory Response: Edema

Functions of the surge of exudate
• Moves foreign material into lymphatic
vessels
• Delivers clotting proteins to form a
scaffold for repair and to isolate the
area
Innate defenses
Tissue injury
Internal defenses
Release of chemical mediators
(histamine, complement,
kinins, prostaglandins, etc.)
Release of leukocytosisinducing factor
Leukocytosis
(increased numbers of white
blood cells in bloodstream)
Initial stimulus
Vasodilation
of arterioles
Increased capillary
permeability
Local hyperemia
(increased blood
flow to area)
Capillaries
leak fluid
(exudate formation)
Attract neutrophils,
monocytes, and
lymphocytes to
area (chemotaxis)
Leukocytes migrate to
injured area
Margination
(leukocytes cling to
capillary walls)
Physiological response
Signs of inflammation
Leaked protein-rich
fluid in tissue spaces
Result
Heat
Redness
Locally increased
temperature increases
metabolic rate of cells
Pain
Swelling
Possible temporary
limitation of
joint movement
Leaked clotting
proteins form interstitial
clots that wall off area
to prevent injury to
surrounding tissue
Temporary fibrin
patch forms
scaffolding for repair
Diapedesis
(leukocytes pass through
capillary walls)
Phagocytosis of pathogens
and dead tissue cells
(by neutrophils, short-term;
by macrophages, long-term)
Pus may form
Area cleared of debris
Healing
Figure 20.3
Phagocyte Mobilization


Neutrophils first, they become activated
phagocytes
Macrophages follow to inflamed sites
Phagocyte Mobilization

Steps for phagocyte mobilization
1.
2.
3.
4.
Leukocytosis: release of neutrophils from bone
marrow in response to leukocytosis-inducing
factors from injured cells
Margination: neutrophils cling to the walls of
capillaries in the inflamed area
Diapedesis of neutrophils
Chemotaxis: inflammatory chemicals
(chemotactic agent) promote positive chemotaxis
of neutrophils
Innate
defenses
Internal
defenses
Inflammatory
chemicals
diffusing
from the
inflamed site
act as chemotactic
agents.
Leukocytosis.
Neutrophils enter blood
from bone marrow.
1
Margination.
Neutrophils cling
to capillary wall.
2
Chemotaxis.
Neutrophils
follow chemical
trail.
4
Capillary wall
Basement
membrane
Endothelium
Diapedesis.
Neutrophils flatten and
squeeze out of capillaries.
3
Figure 20.4
Antimicrobial Proteins

Interferons (IFNs) and complement
proteins
• Attack microorganisms directly
• Hinder microorganisms’ ability to reproduce
Interferons



Viral-infected cells are activated to
secrete IFNs
IFNs enter neighboring cells
Neighboring cells produce antiviral
proteins that block viral reproduction
Innate defenses
Virus
Viral nucleic acid
1 Virus
enters cell.
Internal defenses
New viruses
5 Antiviral
proteins block
viral
reproduction.
2 Interferon
genes switch on.
DNA
Nucleus
mRNA
4 Interferon
3 Cell produces
interferon
molecules.
Interferon
Host cell 2
Host cell 1
Binds interferon
Infected by virus; from cell 1; interferon
makes interferon; induces synthesis of
is killed by virus
protective proteins
binding
stimulates cell to
turn on genes for
antiviral proteins.
Figure 20.5
Interferons

Functions

Genetically engineered IFNs for
• Anti-viral
• Reduce inflammation
• Activate macrophages and mobilize NK cells
• Antiviral agents against hepatitis and genital
•
warts virus
Multiple sclerosis treatment
Complement





~20 blood proteins that circulate in an inactive form
Include C1–C9, factors B, D, and P, and regulatory
proteins
Major mechanism for destroying foreign substances
Amplifies all aspects of the inflammatory response
Kills bacteria and certain other cell types by cell
lysis
Enhances both nonspecific and specific defenses
Complement Activation

Two pathways
1. Classical pathway
•
•
Antibodies bind to invading organisms
C1 binds to the antigen-antibody complexes
(complement fixation)
2. Alternative pathway
•
Triggered when activated C3, B, D, and P
interact on the surface of microorganisms
Complement Activation

Activated complement
• Enhances inflammation
• Promotes phagocytosis
• Causes cell lysis
• C3b initiates formation of a membrane attack
complex (MAC)
• MAC causes cell lysis by inducing a massive
influx of water
• C3b also causes opsonization, and C3a
causes inflammation
Classical pathway
Antigen-antibody complex
+
complex
Opsonization:
coats pathogen
surfaces, which
enhances phagocytosis
Insertion of MAC and cell lysis
(holes in target cell’s membrane)
Alternative pathway
Spontaneous activation
+
Stabilizing factors (B, D, and P)
+
No inhibitors on pathogen
surface
Enhances inflammation:
stimulates histamine release,
increases blood vessel
permeability, attracts
phagocytes by chemotaxis,
etc.
Pore
Complement
proteins
(C5b–C9)
Membrane
of target cell
Figure 20.6
Fever



Systemic response to invading
microorganisms
Leukocytes and macrophages exposed
to foreign substances secrete pyrogens
Pyrogens reset the body’s thermostat
upward
Fever


High fevers are dangerous because heat
denatures enzymes
Benefits of moderate fever
• Causes the liver and spleen to sequester iron
•
and zinc (needed by microorganisms)
Increases metabolic rate, which speeds up
repair
Adaptive Defenses

The adaptive immune (specific defense)
system
• Protects against infectious agents and
•
•
abnormal body cells
Amplifies the inflammatory response
Activates complement
Adaptive Defenses


•
•
•
Adaptive immune response
Is specific
Is systemic
Has memory
Two separate overlapping arms
1. Humoral (antibody-mediated) immunity
2. Cellular (cell-mediated) immunity
Antigens


Substances that can mobilize the
adaptive defenses and provoke an
immune response
Most are large, complex molecules not
normally found in the body (nonself)
Complete Antigens

Important functional properties
• Immunogenicity: ability to stimulate proliferation
•

of specific lymphocytes and antibodies
Reactivity: ability to react with products of
activated lymphocytes and antibodies released
Examples: foreign protein,
polysaccharides, lipids, and nucleic acids
Haptens (Incomplete Antigens)





Small molecules (peptides, nucleotides,
and hormones)
Not immunogenic by themselves
Are immunogenic when attached to body
proteins
Cause the immune system to mount a
harmful attack
Examples: poison ivy, animal dander,
detergents, and cosmetics
Antigenic Determinants



Certain parts of an entire antigen that are
immunogenic
Antibodies and lymphocyte receptors bind to
them
Most naturally occurring antigens have
numerous antigenic determinants that
• Mobilize several different lymphocyte populations
• Form different kinds of antibodies against it

Large, chemically simple molecules (e.g.,
plastics) have little or no immunogenicity
Antibody A
Antigenbinding
sites
Antigenic determinants
Antigen
Antibody B
Antibody C
Figure 20.7
Self-Antigens: MHC Proteins



Protein molecules (self-antigens) on the
surface of cells
Antigenic to others in transfusions or
grafts
Example: MHC proteins
• Coded for by genes of the major
histocompatibility complex (MHC) and are
unique to an individual
MHC Proteins

Classes of MHC proteins
• Class I MHC proteins, found on virtually all body
•


cells
Class II MHC proteins, found on certain cells in
the immune response
MHC proteins display peptides (usually selfantigens)
In infected cells, MHC proteins display fragments
of foreign antigens, which help mobilize
Cells of the Adaptive Immune
System

Two types of lymphocytes
• B lymphocytes (B cells)—humoral immunity
• T lymphocytes (T cells)—cell-mediated
immunity

Antigen-presenting cells (APCs)
• Do not respond to specific antigens
• Play essential auxiliary roles in immunity