The Immune System
An organisms’ protection
from Pathogens
Video
Pathogen
Any infectious agent that causes disease
• Bacteria
•Viruses
•Fungi
•Protists/parasites
2 Divisions of Immunity in Humans
and Other Mammals
I. Innate Immunity – “Non-Specific”
This defense is not concerned with ‘what’ the pathogen is.
This system merely prevents the pathogen from
entering the body or destroys it before identifying it.
It shoots first and asks questions later
Innate Immunity involves several layers of defense:
A. Barrier Defenses
B. Inflammatory Response
C. Cellular Defenses
D. Natural Killer Cells
E. Antimicrobial Defenses
A. Barrier Defenses
1.
2.
3.
4.
5.
Epidermis – impenetrable barrier
Oil and Sweat have low pH
Resident Flora – your own bacteria
Mucous Membrane – mucus and cilia
Lysozyme
Skin



Epidermis an impenetrable barrier to
pathogens
Oil and Sweat have a low pH that
reduces pathogen growth
Resident Flora your own colonies of
bacteria that live on your skin out-competes with harmful bacteria for space
Mucous Membranes


Un-keritonized skin of the mocous
membranes have a different layer of defense
MUCUS and CILIA
Lysozyme
B. Cellular Defenses
Microbes/antigens
PHAGOCYTIC CELL
Leukocytes – phagocytic
white blood cells have surface Vacuole
receptors that detect typical
pathogen compounds called
antigens.
Lysosome
containing
enzymes
EXTRACELLULAR
Lipopolysaccharide
FLUID
Helper
protein
Groups of
pathogens are
recognized by
TLR, Toll-like
receptors
TLR4
WHITE
BLOOD
CELL
Flagellin
TLR5
VESICLE
CpG DNA
TLR9
TLR3
Inflammatory
responses
A white blood cell
engulfs a microbe, then
fuses with a lysosome to
destroy the microbe
There are different types of
phagocytic cells:




Neutrophils engulf and destroy microbes
Macrophages are part of the lymphatic
system and are found throughout the body
Eosinophils discharge destructive enzymes
Dendritic cells stimulate development of
acquired immunity
Fig. 43-7
Interstitial fluid
Adenoid
Adenoid
Tonsil
Tonsil
Blood
Blood
capillary
capillary
Lymph
Lymph
nodes
nodes
Spleen
Spleen
Peyer’s patches
Peyer’s
patches
(small
intestine)
(small intestine)
Appendix
Appendix
Lymphatic
vessels
Tissue
Tissue
cells
cells
Lymph
Lymph
node
node
Lymphatic
Lymphatic
vessel
vessel
Masses of
Masses of
defensive
cells
defensive cells
C. Antimicrobial Proteins



Peptides and proteins function in innate
defense by attacking microbes directly or
impeding their reproduction
Interferon proteins provide innate defense
against viruses and help activate macrophages
About 30 proteins make up the complement
system, which causes lysis of invading cells and
helps trigger inflammation
D. Inflammatory Responses



Following an injury, mast cells release
histamine, which promotes changes in blood
vessels; this is part of the inflammatory
response
These changes increase local blood supply and
allow more phagocytes and antimicrobial
proteins to enter tissues
Pus, a fluid rich in white blood cells, dead
microbes, and cell debris, accumulates at the
site of inflammation
Fig. 43-8-3
D. Inflammatory Responses
Pathogen
Mast cell
Splinter
Chemical Macrophage
Signals
(Ligand)
Capillary
Red blood cells Phagocytic cell
Fluid
Phagocytosis


Fever is a systemic inflammatory
response triggered by pyrogens
released by macrophages, and toxins
from pathogens
Septic shock is a life-threatening
condition caused by an overwhelming
inflammatory response
II.
Acquired immunity
lymphocyte receptors provide pathogen-specific recognition



Lymphocytes- are white blood cells that
recognize and respond to antigens, foreign
molecules.
Lymphocytes that mature in the thymus above
the heart are called T cells, and those that
mature in bone marrow are called B cells
Lymphocytes have immunological memory.
Fig. 43-9
B cells and T cells have receptor proteins
that can recognize and bind to antigens
Antigenbinding
site
Antigenbinding site
Antigenbinding
site
Plasma
membrane
B cell
(a) B cell receptor
Cytoplasm of B cell
Cytoplasm of T cell
(b) T cell receptor
T cell
Antibody Genes
V
V
Heavy chain
V
D
D
J
J
J
J
C
Gene components
scattered through
one chromosome
Light chain
V
D
J
C
Rearranged
gene
components
encoding a
heavy chain
Antigenbinding region
Constant
region
Assembled
antibody
molecule
Markers of Self:
Major Histocompatibility Complex
Viral
infection
Antigenic
peptide
Antigenic
peptide
Antigenic
peptide
MHC
Class I
MHC
Class I
MHC
Class II
Antigen-presenting cell
uses MHC Class I or II
Infected cell
Cell
membrane
Body Cell with “Self-Markers called MHC
B Cells
Antigen-specific
B cell receptor
Class II MHC and
processed antigen
are displayed
Antigen
B cell
bacteria
Antibodies
(Immunoglobins)
Cytokines (Lymphokines) Plasma cell
LIGAND
Activated
helper T cell
T Cells
Resting helper T cell
Resting cytotoxic T cell
Activated when they encounter
infected cells that are
presenting antigens
Cytokines
Released by
Helper T-Cells
Granule w/
destructive
enzymes
Activated helper T cell
Activated killer cell
Killer Cells: Cytotoxic Ts
Killer cell
Surface contact
Target cell
Target-oriented
granules



T cells bind to antigen fragments
presented on a host cell
These antigen fragments are bound to
cell-surface proteins called MHC
molecules
MHC molecules are so named because
they are encoded by a family of genes
called the major histocompatibility
complex
The Role of the MHC



In infected cells, MHC molecules bind and
transport antigen fragments to the cell
surface, a process called antigen
presentation
A nearby T cell can then detect the
antigen fragment displayed on the cell’s
surface
Depending on their source, peptide
antigens are handled by different classes
of MHC molecules
Fig. 43-12
Infected cell
Microbe
Antigenpresenting
cell
1 Antigen
associates
with MHC
molecule
Antigen
fragment
Antigen
fragment
1
1
Class I MHC
molecule
T cell
receptor
(a)
2
2
Cytotoxic T cell
Class II MHC
molecule
T cell
receptor
2 T cell
recognizes
combination
(b)
Helper T cell
Activation of B Cells to Make Antibody
Circulating antibody
Antigen
Antigen-specific
B cell receptor
Antigen
Class II MHC
B cell
Antigenpresenting cell
Antigen is
processed
Class II MHC Antigen-presenting cell
and
processed
antigen are
displayed
Cytokines
(LIGAND)
Activated
helper T cell
Antibodies
Plasma cell
Fig. 43-14
Antigen molecules
B cells that
differ in
antigen
specificity
Antigen
receptor
Animation: Role of B Cells
Antibody
molecules
Clone of memory B cells
Clone of plasma cells
Fig. 43-16
Humoral (antibody-mediated) immune response
Cell-mediated immune response
Key
Antigen (1st exposure)
+
Engulfed by
Gives rise to
Antigenpresenting cell
+
Stimulates
+
+
B cell
Helper T cell
+
Cytotoxic T cell
+
Memory
Helper T cells
+
+
+
Antigen (2nd exposure)
Plasma cells
Memory B cells
+
Memory
Cytotoxic T cells
Active
Cytotoxic T cells
Secreted
antibodies
Defend against extracellular pathogens by binding to antigens,
thereby neutralizing pathogens or making them better targets
for phagocytes and complement proteins.
Defend against intracellular pathogens
and cancer by binding to and lysing the
infected cells or cancer cells.
Fig. 43-17
Antigenpresenting
cell
Peptide antigen
Bacterium
Class II MHC molecule
CD4
TCR (T cell receptor)
Cytokines
Helper T cell
Humoral
immunity
(secretion of
antibodies by
plasma cells)
+
+
+
B cell
+
Cytokines
Cytotoxic T cell
Animation: Helper T Cells
Cell-mediated
immunity
(attack on
infected cells)
Cytotoxic T Cells: A Response to
Infected Cells


Cytotoxic T cells are the effector cells in
cell-mediated immune response
The activated cytotoxic T cell secretes
proteins that destroy the infected target
cell
Animation: Cytotoxic T Cells
Fig. 43-18-3
Released cytotoxic T cell
Cytotoxic T cell
Perforin
Granzymes
CD8
TCR
Class I MHC
molecule
Target
cell
perforin
granzymes
Pore
Peptide
antigen
Dying target cell
B Cells: A Response to
Extracellular Pathogens



The humoral response is characterized by
secretion of antibodies by B cells
Activation of B cells is aided by cytokines
and antigen binding to helper T cells
Clonal selection of B cells generates
antibody-secreting plasma cells, the
effector cells of humoral immunity
Fig. 43-19
Antigen-presenting cell
Bacterium
Peptide
antigen
Class II MHC
molecule
TCR
CD4
B cell
+
Cytokines
Clone of plasma cells
Secreted
antibody
molecules
Endoplasmic
reticulum of
plasma cell
Helper T cell
Activated
helper T cell
Clone of memory
B cells
2 µm
The Role of Antibodies in
Immunity



Neutralization occurs when a pathogen can no
longer infect a host because it is bound to an
antibody
Opsonization occurs when antibodies bound to
antigens increase phagocytosis
Antibodies together with proteins of the
complement system generate a membrane
attack complex and cell lysis
Animation: Antibodies
Fig. 43-21
Viral neutralization
Virus
Opsonization
Activation of complement system and pore formation
Bacterium
Antibodies bound
to antigens on
viruses can
neutralize the virus
Complement proteins
Macrophage
Binding of antibodies toFormation
bacteria
of
membrane
Promotes phagocytosisattack
of the
complex
Bactria by Macrophages Flow of water
and ions
Pore
Foreign
cell
Following activation the attack complex
forms
poresof
in antibodies
the foreigntocell’s
Binding
antigens on the
membrane
water
and ions to
surface ,ofallowing
a foreign
cell activates
a
rushcomplex
in.
system.
The Cell swells and eventually lyses.
Evolution and Immunity
VIDEO 1
VIDEO 2