1
Q
What are the three lines of defense in the immune system?
A
First line – Physical & chemical barriers
Second line – Non-specific internal defenses
Third line – Specific (adaptive) immune response
2
Q
What makes up the first line of defense?
A
- Intact skin
- Mucous membranes and their secretions
- Chemical factors (e.g., enzymes, pH)
- Normal flora (beneficial microbes)
3
Q
What is the function of the first line of defense?
A
To prevent pathogens from entering the body — acts as a physical and chemical barrier.
4
Q
When does the second line of defense activate?
A
It comes into play once the first line of defense is breached.
5
Q
What are the main components of the second line of defense?
A
Phagocytes (neutrophils, macrophages)
Natural killer (NK) cells
Inflammation
Fever
Antimicrobial substances (like interferons, complement proteins)
6
Q
What is the function of the second line of defense?
A
To destroy invaders and slow infection while the specific immune response prepares.
7
Q
What are the main components of the third line of defense?
A
Specialized lymphocytes (T cells and B cells)
Antibodies
8
Q
When does the third line of defense activate?
A
It is initiated if the non-specific (innate) immune system fails.
9
Q
What makes the third line of defense unique?
A
It is specific to each individual and targets particular pathogens (adaptive immunity).
10
Q
Which line of defense is common to all humans?
A
The first and second lines of defense (innate immunity).
11
Q
Which line of defense is unique to each individual?
A
The third line of defense (adaptive immunity).
12
Q
What are physical factors in the immune system?
A
Barriers that prevent pathogen entry or processes that remove microbes from body surfaces.
13
Q
What is the structure and function of the skin as a physical barrier?
A
Thick layer of tightly packed epithelial cells with keratin
Dryness and periodic shedding remove microbes
Acts as a tough, continuous barrier against entry
14
Q
How do mucous membranes defend the body?
A
Mucus traps microorganisms
Examples:
* Mucociliary escalator – moves mucus up and out of airways
* Lacrimal apparatus – produces tears to wash eyes
15
Q
What is the function of endothelial cells?
A
Line blood and lymphatic vessels
Example: Blood-brain barrier prevents microbes from reaching the brain and spinal cord
16
Q
What role does hair play in defense?
A
Traps and prevents attachment of microbes to mucous membranes (e.g., in nose or ears)
17
Q
What do secretions do as a physical defense?
A
Wash away microbes from exposed surfaces
Examples: Saliva, tears, urine, earwax (cerumen), vomit, diarrhea
18
Q
What are chemical factors in the immune system?
A
Substances produced by the body that inhibit or kill microbial growth.
19
Q
What do sebaceous glands secrete, and how does it protect the body?
A
Sebum (oily substance) from sebaceous glands
Low pH and fatty acids inhibit microbial growth on the skin
20
Q
How does sweat act as a chemical defense?
A
Contains lysozyme (breaks down bacterial cell walls)
Has salt and dermicidin (antimicrobial peptide)
Creates an environment hostile to microbes
21
Q
What protective role do gastric juice, vaginal secretions, and urine share?
A
All have low pH, which inhibits bacterial growth
Examples: Lactic acid in vaginal secretions; acidic urine and stomach acid
22
Q
How does saliva defend against microbes?
A
Contains:
* Low pH
* Lysozyme
* Lactoperoxidase (produces antimicrobial compounds)
* Lactoferrin (binds iron, starving bacteria)
* IgA antibodies (neutralize pathogens)
23
Q
What role do digestive enzymes play in chemical defense?
A
Break down pathogens in the digestive tract
Include proteases, lipases, and amylase
24
Q
What is normal flora?
A
Normal (resident) microbes that naturally live on or in the body and help protect against pathogens.
25
How do normal flora protect the body from pathogens?
Through competitive inhibition — they outcompete pathogens for space and nutrients.
26
What antimicrobial molecules do normal flora produce?
Bacteriocins – proteins that inhibit other bacteria
Lactic acid – lowers pH to create an unfavourable environment for pathogens
27
What are the two main protective actions of normal flora?
Produce antimicrobial substances (bacteriocins, lactic acid)
Occupy space and consume nutrients to prevent pathogen colonization
28
What type of cell is a Natural Killer (NK) cell?
A type of lymphocyte (agranulocyte) that is part of the innate immune system.
29
What is the main function of NK cells?
They recognize and kill infected or tumour cells in the body.
30
How do NK cells identify their targets?
They detect cells displaying abnormal or foreign antigens on their surface.
31
What toxic compounds do NK cells release to destroy target cells?
Back:
Perforin – forms pores in the target cell membrane
Granzymes – protein-digesting enzymes that enter through the pores and trigger apoptosis (cell suicide)
32
What is apoptosis in the context of NK cell activity?
A controlled process of programmed cell death used to eliminate infected or cancerous cells without causing inflammation.
33
What is phagocytosis?
The ingestion of microbes or particles by certain white blood cells called phagocytes.
34
What are phagocytes?
White blood cells that engulf and digest invading microorganisms or debris.
35
Which granulocyte is a phagocyte?
Neutrophils — they are the first responders to infection and quickly engulf pathogens.
36
Which agranulocytes perform phagocytosis?
Monocytes (inactive in blood) that mature into macrophages once they enter tissues
37
What are the two types of macrophages?
Resident macrophages – stay fixed in specific tissues (e.g., liver, lungs)
Wandering macrophages – move throughout tissues to locate and destroy pathogens
38
Step 1 — What happens during Chemotaxis?
Phagocytes are attracted to the site of infection by chemokines or microbial products.
➡️ Acts like a “chemical signal” guiding the phagocyte toward the pathogen.
39
Step 2 — What happens during Adherence?
Phagocyte’s PRRs (Pattern Recognition Receptors) attach to PAMPs (Pathogen-Associated Molecular Patterns) on the microbe’s surface.
➡️ Ensures the phagocyte recognizes the invader.
40
Step 3 — What happens during Ingestion?
The microbe is engulfed into a vesicle called a phagosome inside the phagocyte.
41
Step 4 — What happens during Digestion?
The phagosome fuses with a lysosome to form a phagolysosome.
Digestive enzymes break down the microbe.
42
Step 5 — What happens during Waste Removal?
Indigestible material remains in a residual body and is excreted from the cell
43
What do the abbreviations PAMP and PRR stand for?
PAMP: Pathogen-Associated Molecular Pattern (on microbes)
PRR: Pattern Recognition Receptor (on phagocytes)
44
What is inflammation?
A response to tissue injury designed to confine and destroy pathogens and begin tissue repair.
45
What are the main goals of inflammation?
1. Contain the infection
2. Eliminate the pathogen
3. Initiate tissue repair
46
What are the classic signs of inflammation?
Redness (rubor)
Pain (dolor)
Heat (calor)
Swelling (tumor)
Sometimes loss of function (functio laesa)
47
Why does inflammation cause redness and heat
Due to increased blood flow (vasodilation) to the injured area.
48
What are the three main steps of inflammation?
1.Vasodilation & increased permeability
2. Phagocyte migration & phagocytosis
3. Tissue repair
49
Step 1 — What happens during vasodilation and increased permeability?
Mast cells and resident macrophages recognize pathogens and release histamine and chemokines.
Blood vessels widen and become leaky, allowing immune cells and plasma to move into tissues.
This process is called recruitment — defensive substances leave the blood and enter the injured area.
50
What is the purpose of vasodilation and increased permeability?
To deliver phagocytes, clotting factors, and antimicrobial proteins to the site of injury or infection.
51
Step 2 — What happens during phagocyte migration and phagocytosis?
Phagocytes (mainly neutrophils and macrophages) migrate to the site within about an hour.
They engulf and destroy microbes and dead tissue (the removal phase).
Accumulation of dead cells, bacteria, and fluids forms pus.
52
Step 3 — What happens during tissue repair?
Begins once all harmful substances are removed or neutralized.
White blood cells release factors that stimulate fibroblasts to rebuild tissue.
Chronic inflammation occurs if pathogens persist, possibly forming granulomas.
53
What is a cytokine storm?
An excessive release of cytokines causing severe inflammation that can damage body tissues.
54
What part of the brain regulates body temperature?
The hypothalamus, which normally sets body temperature around 37°C.
55
What causes a fever to occur?
Pathogens and cytokines (pyrogens) trigger the release of prostaglandins, which reset the hypothalamus to a higher temperature.
56
How does the body increase temperature during a fever?
Shivering
Increased metabolic rate
Vasoconstriction (reduces heat loss)
57
What is the purpose of a fever?
Inhibits bacterial growth
Speeds up metabolic reactions for faster tissue repair
Increases T cell production
Enhances interferon activity (antiviral defense)
58
What are potential complications of high fever?
Tachycardia (rapid heart rate)
Dehydration
Acidosis
Seizures (in young children)
Death (if too high or prolonged)
59
What are the four main types of antimicrobial substances in the second line of defense?
a) Complement system
b) Interferons (cytokines)
c) Iron-binding proteins (acute phase proteins)
d) Antimicrobial peptides
60
What is the complement system?
A group of >30 plasma proteins that work together to destroy microbes and enhance immune responses.
61
What is the central protein in the complement system?
C3, which triggers multiple defense reactions once activated.
62
How does the complement system work?
It acts in a cascade — one reaction activates another, creating an amplified response.
63
What can initiate the complement cascade?
Microbial surface molecules
Antibodies bound to antigens
Spontaneous activation of complement proteins
64
What are the three main effects of complement activation?
1️⃣ Inflammation – attracts immune cells
2️⃣ Cytolysis – forms a membrane attack complex (MAC) that punctures microbe membranes
3️⃣ Opsonization – coats pathogens to make them easier for phagocytes to engulf
65
What is an easy mnemonic for complement functions?
Itchy Octopus Cycles → Inflammation, Opsonization, Cytolysis
66
What are interferons (IFNs)?
A class of cytokines produced by body cells during pathogen infection — they help coordinate immune responses and inhibit viral replication.
67
What does Gamma Interferon (IFN-γ) do?
Produced by lymphocytes (T helper cells)
Activates neutrophils and macrophages to kill bacteria
68
What do Alpha and Beta Interferons (IFN-α, IFN-β) do?
Produced by virus-infected host cells
Diffuse to uninfected neighboring cells
Trigger these cells to produce antiviral proteins (AVPs) that block viral replication
69
How are interferons used as drugs?
Produced using recombinant DNA technology
Examples:
* Betaferon (IFN-β) – treats multiple sclerosis
* Intron A (IFN-α) – treats cancers (e.g., Kaposi sarcoma)
70
What are limitations or side effects of interferon therapy?
Not a cure — requires long-term treatment
Side effects: headache, fever, nausea, weight loss
May cause organ toxicity (heart, liver, kidneys, red bone marrow)
71
Why are iron-binding proteins important in immunity?
They isolate free iron to limit bacterial growth, since both human and microbial cells need iron for metabolism.
72
What are the main iron-binding proteins and their locations?
Lactoferrin – tears
Transferrin – blood & tissue fluids
Ferritin – liver, spleen, red bone marrow
Hemoglobin – red blood cells
73
How do pathogens counter iron-binding defenses?
They produce siderophores, molecules that steal iron from host proteins
74
What effect does fever have on iron-binding proteins?
Fever increases production and release of these iron-sequestering proteins
75
What are antimicrobial peptides (AMPs)?
Short peptides (12–50 amino acids) made by many cells in response to microbial antigens; have a broad-spectrum antimicrobial activity.
76
What are the modes of action of AMPs?
Form pores in microbial membranes
Inhibit cell wall synthesis
Destroy DNA/RNA
77
Give examples of antimicrobial peptides and their functions.
Defensins – from neutrophils; damage bacterial & fungal membranes
Dermcidin – found in sweat; kills microbes on skin
Bacteriocins – made by E. coli in microbiome; kill harmful bacteria in colon
78
What is adaptive (specific) immunity?
targets specific pathogens, has memory, and responds stronger upon re-exposure.
79
What happens during the primary immune response?
Occurs on first exposure to a pathogen
Response is slow, taking time for specific B cells and T cells to activate
80
What happens during the secondary immune response?
Occurs on second exposure to the same pathogen
Response is faster and more intense
Pathogen is neutralized before disease develops
81
Why is the secondary immune response stronger?
Because of memory cells formed during the first exposure that quickly recognize the antigen.
82
What does it mean that the secondary response is antigen-specific?
The memory response is directed only toward the same antigen that triggered the first response.
83
What are antigens?
Unique markers on pathogen surfaces that trigger an immune response (“antibody-generating” substances).
84
Where can antigens be found?
On structures such as:
Cell wall
Flagella, fimbriae, pili
Capsule
Capsid or viral spikes
Viral genome
They can also come from non-microbial sources, like allergens or transplanted tissue.
85
What is an epitope?
A very specific region on an antigen where an antibody, B cell receptor, or T cell receptor binds.
86
How are antigens different from PAMPs?
Antigens are highly specific to each pathogen, while PAMPs are general molecular patterns common to many microbes.
87
What are haptens?
Small molecules too tiny to trigger an immune response on their own.
88
How can haptens become immunogenic?
When they bind to a larger carrier molecule, they can be recognized by the immune system.
89
What are the main functions of the lymphatic system?
1️⃣ Immunity – production and activation of immune cells
2️⃣ Fluid regulation – returns interstitial fluid to bloodstream
3️⃣ Transport of fats – absorbs and transports lipids from the digestive tract
90
What components make up the lymphatic system?
Organs, vessels, and lymph fluid (similar to interstitial fluid)
Includes: red bone marrow, thymus, spleen, lymph nodes, and diffuse lymphatic tissue
91
Where do most adaptive immune responses occur?
In secondary lymphoid organs such as the spleen, lymph nodes, and lymphatic tissues.
92
Where are lymphoid stem cells formed?
In the red bone marrow.
93
Where do T cells mature?
They migrate to the thymus, where they mature into T lymphocytes (T cells).
94
Where do B cells mature?
They remain in the bone marrow, where they mature into B lymphocytes (B cells).
95
What does it mean that T and B cells are “mature but naïve”?
They are ready to recognize a specific antigen but have not yet encountered it.
96
What does it mean that each T and B cell is “tuned” to a specific antigen?
Each lymphocyte has unique receptors that match only one specific antigen — like a lock-and-key system.
97
What are the two branches of adaptive immunity?
1️⃣ Cellular (cell-mediated) immunity
2️⃣ Humoral (antibody-mediated) immunity
98
What does cellular immunity target?
Foreign material inside body cells (e.g., viruses, cancer cells)
Involves T lymphocytes (T cells)
99
What does humoral immunity target?
Foreign material in body fluids (e.g., bacteria, toxins in plasma)
Involves B lymphocytes (B cells)
100
Do cellular and humoral immunity occur separately?
No — they occur simultaneously and support each other during an immune response.
101
Which cells are involved in cellular immunity (the “good side”)?
Back:
Antigen-presenting cells (APCs) like macrophages & dendritic cells
T cells:
* Helper T cells (CD4⁺)
* Cytotoxic T cells (CD8⁺)
* Memory T cells
102
Which cells represent the “evil side” targeted by cellular immunity?
Abnormal or infected body cells, such as:
Cancer cells
Cells invaded by microbes (e.g., viruses or bacteria)
103
What cell types can act as antigen-presenting cells (APCs)?
Macrophages, dendritic cells, and B cells — they phagocytose pathogens and present antigens on MHC-II molecules.
104
What are the three steps required to activate a naïve helper T cell?
Back:
1. TCR binds antigen on the APC
2. CD4 binds MHC-II on the APC
3. Cytokines (from APC + T cell) complete activation
105
What cytokine example helps complete helper T cell activation?
TNF-α (Tumor Necrosis Factor alpha)
106
After activation, what happens to helper T cells?
They undergo clonal expansion and differentiate into:
TH1 cells → activate cytotoxic T cells & enhance innate defenses (macrophages, NK, neutrophils)
TH2 cells → activate B cells
Memory helper T cells → long-lived cells for secondary response
107
What are the three steps to activate a naïve cytotoxic T cell?
1. TCR binds antigen on infected/abnormal cell
2. CD8 binds MHC-I on that cell
3. Cytokines (from itself + helper T cells) ensure full, long-lasting activation
108
What is the role of activated cytotoxic T cells?
Specialized cytotoxic T cells: release perforin (pores) & granzymes (apoptosis)
Memory cytotoxic T cells: remain in lymphoid tissue → rapid secondary response
109
What is apoptosis?
Programmed cell death — cleanly destroys infected or abnormal cells without inflammation.
110
What are the key players in humoral immunity?
Activated Helper T cells (TH2)
B cells
Antibodies
111
What is the target of humoral immunity?
Free/extracellular antigens (pathogens or toxins in body fluids).
112
What is an antibody?
A Y-shaped defensive protein, not a cell; tags pathogens for destruction.
113
What do the arms and stem of an antibody do?
Arms (variable regions): bind specific antigen
Stem (constant region): determines antibody class (IgG, A, M, E, D)
114
Match antibody structure with form:**
Monomer: IgG, IgD, IgE
Dimer: IgA
Pentamer: IgM
115
Which antibody classes have special roles?
IgA: in secretions (saliva, tears, mucus, milk)
IgM & IgD: coat B cells (surface receptors)
IgE: allergic reactions & parasite defense
IgG: crosses placenta → passive immunity & general functions
116
How do antibodies protect us from pathogens?
1️⃣ Neutralization – block pathogen binding sites or toxins
2️⃣ Agglutination – clump microbes for easy phagocytosis
3️⃣ Opsonization – coat pathogens to enhance phagocytosis
4️⃣ ADCC (Antibody-Dependent Cell-Mediated Cytotoxicity) – recruit NK cells to kill tagged targets
5️⃣ Complement Activation – leads to cell lysis via MAC formation
117
What are the steps in helper T cell-dependent B cell activation?
1️⃣ Antigen binds surface antibody (IgD/M) on inactive B cell
2️⃣ B cell internalizes & displays antigen on MHC-II
3️⃣ Activated TH2 cell binds same antigen → releases cytokines (e.g. IL-4)
➡️ B cell activates (“linked recognition”)
118
What happens after B cell activation?
B cells proliferate (clonal expansion) and differentiate into:
Plasma cells → secrete antibodies (IgM first, then IgG switch)
Memory B cells → long-term immunity
119
What happens in primary vs secondary B cell responses?
Primary: slow, low antibody levels (initial infection)
Secondary: fast, strong IgG response (due to memory cells)
120
How can some B cells be activated without T cell help?
1️⃣ Large antigens (polysaccharides / LPS) bind multiple surface IgD/M on the same B cell
2️⃣ Microbial PAMP binds B cell PRR
➡️ Leads to plasma cells secreting antibodies (weak response)
121
Why is the T-independent B cell response weaker?
Produces fewer plasma cells
No memory B cells formed
122
What is the role of Helper T cells?
“Central orchestrators” — secrete cytokines that coordinate innate & adaptive immunity.
123
What is the role of Cytotoxic T cells?
Destroy infected or abnormal cells by releasing perforin & granzymes to induce apoptosis.
124
What is the role of B cells?
Defend against pathogens & toxins in extracellular spaces by producing antibodies.
125
What is the function of memory cells (B, Helper T, Cytotoxic T)?
Long-lived cells that “remember” specific antigens → enable faster and stronger secondary immune responses.
126
What is the main purpose of vaccination?
To induce a specific primary immune response without causing disease, so that the body can respond rapidly during future exposure.
127
What happens when a vaccinated person encounters the actual pathogen later?
A secondary immune response occurs — memory B and T cells quickly recognize the antigen and destroy it before illness develops.
128
What cells are responsible for the secondary immune response?
Memory B cells (antibody production) and memory T cells (cellular response).
129
How does vaccination protect communities?
Through herd immunity — when most of the population is immune, the disease can’t spread easily and becomes sporadic.
130
Describe the immune response to vaccination.
1️⃣ Primary response – exposure to vaccine antigen; slow rise in antibodies
2️⃣ Memory cell formation – B and T cells store antigen info
3️⃣ Secondary response – rapid, strong antibody and cytotoxic T cell production upon real exposure
131
Are vaccines perfectly safe and effective?
No — but they remain the safest and most effective way to prevent infectious diseases.
132
What are live, attenuated vaccines?
Contain weakened forms of the pathogen that can infect cells but cannot cause disease.
➡️ Strong, long-lasting immunity — often no booster needed.
Examples: MMR (measles, mumps, rubella), nasal spray influenza (children).
133
What are inactivated vaccines?
Contain pathogens that are killed or inactivated — can’t reproduce.
➡️ Booster shots required to maintain immunity.
Examples: Influenza (injection), Hepatitis A.
134
What are subunit vaccines?
Contain only specific microbial fragments (e.g., proteins or capsid parts) that trigger an immune response.
➡️ Safe, but may require boosters.
Examples: Hepatitis B (capsid fragments), HPV vaccine.
135
Why are conjugated vaccines used?
Because polysaccharide antigens (T-independent) don’t produce strong or lasting immunity.
They are linked to a protein to stimulate a T-dependent response and create memory cells.
Example: Haemophilus influenzae type b (Hib) vaccine for meningitis.
136
What are nucleic acid vaccines?
Contain DNA plasmids or mRNA encoding antigen proteins — host cells make the antigen themselves.
➡️ Stimulate both cellular & humoral immunity; may not require boosters.
Examples: COVID-19, Zika, West Nile (horses).
137
Which type of vaccine only works for protein-based antigens?
Nucleic acid (DNA/mRNA) vaccines.
138
What has been the impact of vaccines in Canada?
Dramatic reduction in infectious diseases; many once-common diseases (like polio or measles) are now rare due to immunization programs.
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