Body Defenses and
Immunity
The Lymphatic System
Consists of two semiindependent parts
Lymphatic vessels
Lymphoid tissues and organs
Lymphatic system functions
Transports escaped fluids back
to the blood
Plays essential roles in body
defence and resistance to
disease
Lymphatic Characteristics
Lymph - excess tissue fluid
carried by lymphatic vessels
Properties of lymphatic
vessels
One way system toward the
heart
No pump
Lymph moves toward the
heart
Lymph
Harmful materials that enter
lymph vessels
Bacteria
Viruses
Cancer cells
Cell debris
Lymph Nodes
Filter lymph before it is returned to
the blood
Defense cells within lymph nodes
Macrophages—engulf and destroy
foreign substances
Lymphocytes—provide immune
response to antigens
Lymphatic Vessels
Lymph capillaries
Walls overlap to form
flap-like mini-valves
Fluid leaks into lymph
capillaries
Capillaries are anchored
to connective tissue by
filaments
Higher pressure on the
inside closes mini-valves
Fluid is forced along the
vessel
Types of Defenses
Innate (nonspecific
defenses)
Function the same way
regardless of the
pathogen type
Adaptive (specific)
defenses
Immunity: targets certain
pathogens
Based on body’s ability
to distinguish “self” from
“non-self”
Antigen – molecule that
elicits an immune
response
Nonspecific Defenses
Species resistance
Mechanical barriers
Chemical barriers
Fever
Inflammation
Phagocytosis
Inflammation
Redness from blood
vessel dilation
Swelling from
increased
permeability of
capillaries
Heat from increased
blood flow
Pain
Phagocytosis
“Cell eating”
Performed by
neutrophils and
macrophages that
form from
monocytes
Development of Cells of Immunity
Fetal bone marrow forms
lymphocytes
Lymphocytes
differentiate:
In thymus  T cells
In marrow  B cells
T cells can form B cells
T and B cells circulate in
bloodstream and are
concentrated in lymph
nodes and spleen
T Cell Types
Helper T cells
Stimulate B cells to
produce antibodies
specific to an antigen
Memory T cells
Produced upon initial
exposure to antigen but do
not act until future
exposure to the same
antigen
Cytotoxic T cells
Destroy cancerous or
virally-infected cells using
perforin
Once activated, proliferate
to increase clones
Primary vs. Secondary Immune Responses
Primary Immune
Response
Occurs with first
exposure to an
antigen
Production and
release of antibodies
continues for several
weeks
After primary
response, memory T
and B cells are
produced
Secondary Immune
Response
Occurs with future
exposures to antigen
Faster response time
Primary Immune Response
• Macrophages engulf invaders and present
them to helper T cells in the nearest lymph
node.
Cell-Mediated Immunity
•
T cells attach to
foreign cells and:
a. Secrete toxins that
kill target cells.
b. Secrete growthinhibiting factors
that prevent target
cell growth.
c. Secrete interferon
that inhibits
proliferation of
viruses and tumor
cells.
Antibody-Mediated Immunity
•
•
•
Helper T cells
activate B cells.
B cells produce
clones (plasma cells
and memory B cells)
and antibodies.
Antibodies destroy
antigen.
Adaptive Immune Responses
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Antibody Actions
Directly attach to antigen and destroy it
Label the antigen for destruction by macrophages
Stimulate localized changes that help prevent the
spread of antigens
Secondary Immune Response
Same process as primary immune response
with some exceptions:
Faster response time
Memory B cells produce clones quickly
Allergic Reactions
An immune attack against
a non-harmful substance
May be caused by
inherited tendency to
overproduce IgE
antibodies in response to
certain antigens
IgE triggers the release of
histamine  allergy
symptoms
If reaction is severe 
anaphylactic shock
Types of Immunity
Naturally acquired active
immunity
Develops after a primary immune
response
Artificially acquired active
immunity
Vaccine stimulates a primary
immune response without causing
symptoms
Artificially acquired passive
immunity
Injection of antibodies; susceptibility
may persist
Naturally acquired passive
immunity
Antibodies passed to fetus from
mother; persists 6 months to one
year after birth
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Autoimmunity
When the immune system
attacks “self”
Examples of autoimmune
diseases
Multiple sclerosis - white matter
of brain and spinal cord are
destroyed
Type I diabetes mellitus destroys pancreatic beta cells
that produce insulin
Rheumatoid arthritis - destroys
joints