Transcript ECOLOGY SPRING 2009 - Florida International University

The Immune System
Class
Introduction
Vertebrates have three levels
of defenses
1. The Integumentary System
Skin and mucous
membranes provide first line
of defense
2. Nonspecific (innate)
Immune System
Acts very rapidly after onset
of infection
3. Specific Immune System
Eliminates microbes that
escaped the second line of
defense
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Integumentary system
The skin is the largest organ of the body
Provides a nearly impenetrable barrier, reinforced with
chemical weapons
Oil & sweat glands give skin a pH of 3-5
Lysozyme breaks bacterial cell walls
Also contains many normal flora
Non-pathogenic microorganisms that out-compete pathogenic
ones
Composed of Epidermis, Dermis, and Subcutaneous
layers.
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Mucosal Epithelial Surfaces
The digestive, respiratory and urogenital tracts are lined
by mucous membranes
-Cells secrete mucus which traps microbes
Digestive tract
-Salivary lysozyme; acidic stomach
Respiratory tract
-Ciliary action
Urogenital tract
-Acidic urine
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Nonspecific Immunity
The nonspecific or innate immune system consists of
cellular and chemical devices that respond to any
microbial infection
-The response is quite rapid
Among the most important defenses are three types of
leukocytes (white blood cells)
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Leukocytes
Macrophages
•Large, irregularly shaped
cells
•Kill microbes by
phagocytosis
•Mature from monocytes
that enter tissues from
the blood
Neutrophils
•The most abundant
circulating leukocytes
•First to appear at site of
damage/infection
•Kill microbes by
phagocytosis
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Leukocytes
Natural killer (NK) cells
•Destroy pathogen-infected and cancer
cells by causing programmed cell death or
apoptosis
• Produce perforins and granzymes
• Immune surveillance
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The Inflammatory Response
Inflammation involves several body systems
1. Injured cells release chemical alarms, including
histamine and prostaglandins
2. Cause nearby blood vessels to dilate and increase in
permeability
3. Promote phagocyte accumulation
•Hallmark signs = Redness, warmth, swelling, pain, and
potential loss of function
•Interleukin-1 (IL-1) causes fever.
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The Inflammatory Response
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Complement
The complement system consists of about 30 different
proteins that circulate in the blood in an inactive form
-Upon pathogen encounter, a cascade of activation
occurs
-Some proteins aggregate to form a membrane attack
complex (MAC) on surface of pathogen
Pore
Fluid
Membrane
of invading
microbe
Complement
proteins
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Interferon
Interferons (IFN) are proteins that play a key role in body
defense
-Three major types: IFN-a, IFN-b, IFN-g
-IFN-a and IFN-b are produced by almost all body
cells in response to viral infection
-Induce degradation of viral RNA
-IFN-g is produced only by T-lymphocytes and
natural killer cells
-Protects from infection and cancer
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The Specific Immune System
The scientific study of
immunity began with Edward
Jenner in 1796
Vaccination
-Observed that milkmaids
who had cowpox rarely
experienced smallpox
-Inoculated individuals with
fluid from cowpox vesicles
to protect them from
smallpox
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Antigens
An antigen is a molecule that provokes a
specific immune response
-May be components of microorganisms or
proteins/glycoproteins found on surface of
red blood cells or transplanted tissue cells
A single protein may have many different
antigenic determinants or epitopes
-Each can stimulate a distinct immune
response
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Lymphocytes
Lymphocytes are leukocytes with surface receptors for
antigenic determinants
•Direct an immune response against either the antigen or
the cell that carries it
When a naïve lymphocyte binds a specific antigen for the
first time, it gets activated by a process called clonal
selection
•Produces a clone of cells: some respond immediately,
others are memory cells
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Lymphocytes
B lymphocytes or B cells
•Respond to antigens by secreting antibodies or
immunoglobulins (Ig)
•Participate in humoral immunity
T lymphocytes or T cells
•Regulate other immune cells or directly attack cells that
carry specific antigens
•Participate in cell-mediated immunity
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The Specific Immune System
The four characteristics of the specific, or adaptive,
immune response are:
-1. Specificity
-2. Diversity
-3. Memory
-4. Ability to distinguish self from non-self
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Acquisition of Specific Immunity
Immunity can be acquired in two ways
•Active immunity results from activation of
an individual’s own lymphocytes
• Pathogen infection or vaccination
•Passive
immunity results from obtaining
another individual’s antibodies
• Transfer of maternal antibodies across
placenta
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Organs of the Immune System
All cells are created in a process
called hematopoiesis.
Organs of the immune system
consist of the:
•Primary lymphoid organs
• Bone marrow and thymus
•
Secondary lymphoid organs
• Lymph nodes, spleen, and mucosal-
associated lymphoid tissue (MALT)
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Primary Lymphoid Organs
The bone marrow is site of B cell maturation
•Each B cell has about 105 Ig molecules on its surface, all
with the same specificity
• However, different B cells will have different specificities
•B
cells recognize epitopes directly
•Any lymphocytes that are likely to bind to self-antigens
undergo apoptosis
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Primary Lymphoid Organs
The thymus is the site of T cell maturation
•Each T cell has about 105 identical T-cell
receptors, or TCRs on its surface
• Recognize epitopes only if they are combined with
major histocompatibility complex (MHC) peptides
• Lymphocytes that cannot bind MHCs, or that bind
self-MHC/self-peptide too tightly undergo apoptosis
•T
cells come in two flavors—Cytotoxic T (Tc or
CD8) and Helper T (TH or CD4)
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Immune Responses
The first encounter with a foreign antigen is called the
primary immune response
•Only few B or T cells can recognize antigen
The second encounter is called the secondary immune
response
•This time there is a large clone of memory cells that can
recognize the antigen
• Immune response is more effective
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Amount of antibody
IgM
IgG
Secondary response
Primary response
This interval
may be years
Exposure
to cowpox
Exposure
to cowpox
Time
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Autoimmunity
The acceptance of self cells is known as immune
tolerance
Autoimmune diseases are caused by the failure of
immune tolerance
•Result in activation of autoreactive T cells, and
production of autoantibodies by B cells
• Cause inflammation and organ damage
• Alleviated by corticosteroids and NSAIDs, including aspirin
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