Immunology and Microbiology
Host-Microbe Interactions
Non-specific (Innate) Immunity
First line of defense (Non-specific resistance)
• Physical and Chemical Defenses
• Normal Flora
Second line of defense (Non-specific resistance)
• Phagocytosis
• Inflammation
• Fever
• Antimicrobial substances: Complement,
Third line of defense (Specific Resistance)
• Antibodies
• B cells and T cells
• Detect enemy
Immobilize/kill enemy
Send out alarm
Recruit help
Make environment
unpalatable for
• (process repeats)
• Toll-like receptors on
complement (3 ways)
• Phagocytosis; lysis
• Cytokine release
• Inflammation; fever
Toll-like receptors: What are some evolutionarily
conserved features of microbes that our immune
cells could recognize?
Step 2.
Steps in phagocytosis
Clinical and laboratory features of patients with an
inherited deficiency of neutrophil membrane
complement receptor type 3 (CR3) and the related
membrane antigens LFA-1 and p150,95. Ross GD.
Over the last 3 years a group of more than 20 patients has
been described worldwide who have a similar history of
recurrent bacterial infections and an inherited deficiency
of three related leukocyte membrane surface antigens
known as CR3, LFA-1 and p150,95 (function unknown).
It is believed that the patients with this disease have a
reduced or absent expression of all three antigen family
members on different WBC types. Neutrophils have a
reduced phagocytic response to bacteria and yeast as
well as a reduced ability to migrate into sites of infection.
Adapted from J Clin Immunol. 1986 Mar;6(2):107-13.
The Complement System
• Activated in 3 ways
– Antibody-antigen (augments specific defense)
– Molecules that recognize bacterial sugar polymer
– “Random binding” to cell surfaces (C3b)
• Triggers a cascade
• 3 effects
Effects of complement system
1. Opsonization
2. Membrane Attack Complex (Lysis)
Are G+ or G-ve cells more susceptible to killing by MAC? Why?
Bacterial cell wall structure
Gram-negative and gram-positive bacteria
3. Inflammation
• What else triggers
inflammation? Tissue
• What are features of
• What is the
inflammatory process?
Which of the following statements about
inflammation is false?
A. Vasodilation results in leakage of blood
B. The process can cause damage to host tissue
C. It is always accompanied by a fever
D. The signs of inflammation are redness, swelling,
heat and pain
E. It can be triggered by activating toll-like
receptors (macrophages produce cytokines)
trauma or infection
chemotaxis and diapedesis
Is that pus?
Is inflammation a good thing?
The movement of phagocytes in the direction of an
infection, due to attraction by complement, chemicals
released by microorganisms, and the remnants of
damaged cell membranes is a process called
A) phagocytosis.
B) chemotaxis.
C) diapedesis.
D) cytoadherence.
• Hypothalamus controls
body temp
• Pyrogens resets temp
set point
• Pyrogens: cytokines;
• Unfavourable for
bacterial replication
• Favourable for immune
lymphocyte replication
Interferon and viral infections: a protective alerting system
Complement and immunoglobulins can coat the outer
surface of a microorganism, enhancing phagocytosis.
This process is called
A) chemotaxis.
B) fixation.
C) opsonization.
D) endotoxicity.
E) membrane attack complexing.
Host-Microbe Interactions
1. Innate (Non-specific) Defenses
a. Chemical and Physical;
b. Normal Flora
Host-Microbe Interactions
1. Contact with microbe
a. colonization
(normal flora)
b. defense breached
c. Loss
2. Infection
a. cure, immunity
b. establishment
3. Disease
c. carrier
d. Allergy
All of the following statements are true with regard
innate immunity EXCEPT:
A)These responses are not directed specifically to
the infectious agent.
B)These responses are not affected by prior
exposure to the agent.
C)These responses are inborn.
D)These responses are operating constantly to
prevent the establishment of any infection.
E)All of the above are true with regard to
innate immunity.
First line of defense
An enzyme found in our tears, saliva, serum, and mucus
that degrades the peptidoglycan of the cell wall of Grampositive bacteria is called
A) amylase
B) lysozyme.
C) keratinase.
D) streptokinase.
E) peptidase.
First line of defenseChemical & Physical Barriers
Which of the following substances is produced by the
cells in our body and interferes with the multiplication of
viruses by stimulating the production of antiviral
A) antivirase.
B) interferon.
C) inhibitase.
D) complement.
E) multiplicase.
Which of the following are mechanisms that protect
the respiratory system from infection?
1. mucus
2. mucociliary escalator
3. normal flora
4. lysozyme
5. acidic environment
Normal Flora of Humans
What normal flora?
• A normal human has approximately 1013 body
cells and 1014 individual normal flora!
• microbes that grow on external and internal surfaces
of the body without producing obvious harmful
• Transient microbial flora : only occasionally inhabit
the body.
• Symbiotic relationships: Commensal, Mutualistic,
Body sites that harbor
normal flora
Importance of the normal flora
• Prevent attachment of invading organisms
• Produce antimicrobial substances against
other microbes that are pathogens
• Stimulate immune system
– Cause the production of cross-reacting antibodies
• Significant nutritional source of vitamins
• Cause dental caries and gum disease
How do we acquire microflora?
• During birth & within first 12 hours after
• Breast-fed v bottle-fed
• Contact with people, environment, food.
• Eruption of teeth & introduction of solid food.
What leads to changes in the normal
Antibiotic treatment
Changes in physiology , e.g. estrogen-glycogen
Normal Skin Flora
• 1000 to >1 million/ cm2
• Diphtheroids: G+ rods & cocci, e.g.
Corynebacterium & Proprionobacterium , e.g.
P.acnes (acne)
• facultatively anaerobic, coagulase negative
Staphylococci [carriers of coagulase positive S.
aureus in population]
• Yeasts
First line of defense (Non-specific resistance)
• Physical and Chemical Defenses
• Normal Flora
Second line of defense (Non-specific resistance)
• Phagocytosis
• Inflammation
• Fever
• Antimicrobial substances: Complement, Interferon
Third line of defense (Specific Resistance)
• B cells and T cells
• Antibodies and Humoral Response
• Cellular Mediated Immunity
Genetic Immunodeficiencies
Common Variable Hypogammaglobulinemia: Affecting both males and females and
occurring at any age, this disease is manifest by repeated pyrogenic infections. The
B cells fail to mature to plasma cells. Passive Ig is the common treatment.
Chronic Candidiasis: Fungal infection by nonpathogenic yeast is the manifestation of a
number of afflictions where few mature T cells are present. These patients often
have B cells that respond to T-independent antigens, but Candida albicans (yeast)
stimulates a poor antibody response.
What immune component fails to be made in CVH?
How come a defect in T cells affects antibody response in chronic candidiasis?
The two arms of the adaptive immune system
Antigen: a molecule (or parts of one) that causes antibody generation
The specific region on
an antigen recognized
by an antibody
Antibody Structure
Diversity in antibodies due to variable region
An infinitely large number of possible immunoglobulins;
5 different classes: IgG, IgM, IgA, IgD & IgE
Made by V(D)J recombination
Effects of Antigen-Antibody Interactions
Fig. 16.06
Effects of Antigen-Antibody Interactions-2
Parasites; virally –
infected host cells
NK cells release perforins
and proteases (16.8)
How is the antibody response triggered?
1. T-cell dependent antigens
2. T-cell independent Ags
e.g. polysaccharides, LPS
response of young children to
these antigens is poor
Result: Clonal selection
and expansion of B-cells
A plasma cell
Clonal Expansion
Why is the RER in plasma cells so
Negative selection
Affinity maturation
Class switching: IgM – IgG—IgG /
Formation of memory cells
Memory Cells mediate secondary response and lifelong immunity
Fig. 16.11
Cellular Immunity
Cytotoxic T cells (CD8+)
Eliminates cells infected with virus, intracellular parasite
Helper T cells (CD4+)
Mediates B-cell proliferation; macrophage activation
Both stimulated by dendritic cells (cells of innate immunity)
Both produce cytokines that stimulate own proliferation
T cells activated by dendritic cells
A. Recognition of virally-infected cell by cytotoxic T cell
results in apoptosis
B: Helper T-cell activation and interaction with B-cells
C. Helper T- cells can also
activate macrophages
Fig. 16.19
Activated macrophage: a hungry beast!
•membrane becomes irregular
•increased number of _lysosomes, containing
antimicrobial substances________
•produce nitric oxide
Applications of Principles of Immunity
IgG levels in fetus vs infant
What is passive and what is active?
Principles of Immunization
Natural or artificial
Passive or active
Natural passive , e.g. IgG across the placenta
Artificial passive, use of immune serum
globulin e.g. IgG fraction of donor blood.
• Active involves lymphocytes and confers
lasting protection due to memory
Can you put the types of vaccines listed into categories?
• Induce artificial active immunity
• Preparation of living or inactivated microbe or
virus or their components.
• Adjuvants help to induce better response
• Effective vaccines should be safe, few side
effects, lasting protection, low cost, stable,
easy to administer
• Should induce appropriate specific response
Attenuated immunizing vaccines
Use modified live microbe/virus
Induce infection & mild disease and solid long lasting immunity.
Single dose can induce immunity
Potential for spread to other people helps to develop HERD IMMUNITY
Disadvantages: may cause disease, cannot use in pregnancy, require
• Examples: measles, mumps, rubella, Sabin polio vaccine, (Vaccinia ( for
Inactivated Immunization
• Inactivated by chemical treatment but still antigenic
• Cannot cause infection
• Disadvantages: require several boosters, may cause side
• Whole agent--Use inactivated bacteria or virus
– Examples : Salk polio vaccine, diphtheria & tetanus toxoids
Inactivated Immunization: Subunit
• Use isolated antigens or antigen fragments: a
subunit of the total agent
– bacterial toxin (toxoid), protein subunit,
– e.g vaccines against meningococci, pneumococci,
pertussis, H. influenza
– Recombinant vaccine, e.g. Hepatitis B. Require
several doses.
Future developments & information
• HIV/AIDS, Malaria, cancer
• Use of DNA alone
• Further information: (Children’s Hospital of Philadelphia)
• Developing New Smallpox Vaccines, in EID, vol7, #6, 2001. On
line at
• Testing for the presence of a specific antigen
using specific antibody (antiserum)
• Examples: ELISA blood test for HIV,
home pregnancy test
Mechanisms of Pathogenicity
Attachment and Colonization
Avoidance of Immune System
Toxin Production
Washwater containing Pseudomonas was
sterilized and used to wash cardiac
catheters. Three patients undergoing
cardiac catherization developed fever,
chills, and hypotension. The water and
catheters were sterile. Why did the
patients show these reactions?
Host-Parasite Dynamics
Modes of transmission
Adherence and invasion of host
Growth in/on host
Evade host defense
Damage host
Leave host for reservoir or new host
How do bacterial pathogens overcome host defenses?
bacterial products or structural components that contribute to pathogenicity
or disease
promote bacterial colonization of the host
(infectivity and invasiveness)
obtain nutrients, especially iron
evade immune system
actively harm the host (toxins)
A1. Adherence Factors: Attachment and Colonization
1. Motility
Swim through mucus and towards epithelial cells of mucus membranes
Most microbes that colonize bladder, intestines
are motile (need to reach epithelial cells before flushed out)
Non-motile Vibrio cholerae
mutants are less virulent
2. Pili and fimbrae with adhesins
Pili allow uropathogenic bacteria to
resist flushing and colonize the urethra.
Once bacteria attach, some remain on cell surface
like E. coli and biofilm bacteria.
Others INVADE host cells.
2a. Capsule
Polysaccharide coat is sticky and aids attachment
Also protects cell from inflammatory response
Reduced complement fixation and phagocytosis.
3b. Biofilm: community of encapsulated bacteria
Aid attachment of other bacteria; evade host immune
A2. Invasion
Proteins that induce phagocytosis in non-phagocytic
Bacterium escapes from
vacuole and lives in cytoplasm
Advantage of living
in host cell:
Ready supply of nutrients
Protected environment
Mechanism of Invasion: Type III secretion system
Pore formed through bacterial membrane and wall
and human cell membrane.
Deliver proteins
directly from
bacterial cytoplasm
to human cell
that polymerize
actin filaments.
Invasion of tissues possible too.
Invasion virulence factors that damage tissue
or dissolve materials
e.g. Collagenase breaks down connective tissue
Clostridium perfringens: gangrene
C. Virulence Factors that help evade immune system.
i. Avoid or escape phagocytosis
destroy phagocytes or signals for recruitment
type III secretion of proteins that directly inhibit phagocytosis: Yersinia pestis
•prevent opsonization (Protein A and Fc receptor)
ii) evade specific immune response (antibodies)
Antigenic variability (phase variation):
alter genes that encode surface proteins
like adhesins
Low immunogenicity:
‘Teflon’ pathogens (spirochaetes)
few surface proteins
IgA protease
Viral evasion of cytotoxic T cell and NK cell attack
D. TOXINS: Virulence factors that damage the host.
Endotoxin is only released upon death (lysis) of bacterium
What causes lysis?
LPS, especially when in the blood, can cause a number of pathophysiological changes
such as:
a. fever production
b. inflammation
c. tissue destruction
d. respiratory distress
e. capillary damage (leading to petechial rash, capillary leakage, and hypovolemia
f. intravascular coagulation
g. hypotension
h. decreased cardiac output
i. Septic shock
j. wasting of the body
k. diarrhea (from endotoxin in intestines)
Released from viable bacteria
Gram-negative and gram-positive cells
Most potent toxic substances known
1 ng of botulinum toxin kills 1000 people
Bind to specific receptors on host cells
(more localized effect than endotoxin)
Effect depends on cell type the toxin binds
•Neurotoxins (nervous tissue)
•Cytotoxins (general tissue)
•Enterotoxins (GI tract)
Examples of bacterial exotoxins
Botulinum toxin (neurotoxin)
Blocks release of neurotransmitter
Paralysis results
Cholera toxin (enterotoxin)
Fluid loss results
Genetics of Pathogenicity
can involve many genes, complex.
chromosomal, plasmids, bacteriophage
Transfer of virulence genes via transformation,
transduction, and conjugation.
Virulence factor genes often transferred as groups
called pathogenicity islands: type III secretion system

similar documents