L22 - Killing Pathogens: Extracellular Microbes

Question 1

What key events occur during the initial inflammatory response?

Answer 1

• Chemical mediators are produced (e.g., cytokines).
• Immune cells differentiate, developing killing mechanisms.
• PAMPs (Pathogen-Associated Molecular Patterns) and DAMPs (Damage-Associated Molecular Patterns) are recognized.
• Recognition leads to cytokine release.
• Results in increased vascular permeability to allow immune cells to access the infection site.

Question 2

What happens during DC activating T cells

Answer 2

• dendritic cells scans and take up samples
• presents the molecules to T cells at lymph nodes
• T cells see if they recognised and takes action
• if they detect antigen that they recognised then it starts differentiating

Question 3

What are the 3 T cell types and what do they do

Answer 3

T helper (Th cells) - communicates and coordinates immune response. Express CD4 and recognises extracellular Ag presented by MHC class 2

Regulatory T cells (Treg cells) - turn off immune response. Express CD4 and recognises extracellular Ag presented by MHC class 2

Cytotoxic T cells (CTL) - kill infected cells. Express CD8 and recognise intracellular Ag presented by MHC class 1

Question 4

What happens when T cells are activated?

Answer 4

They divide and differentiate into an effector cell

• T cells that have the required antigens undergo clonal expansion driven by cytokines
• who do they then fight infections?
• they differentiate into their effector phenotype (e.g. killer phenotype)

Question 5

How are diff Th cell subsets defined?

Answer 5

Defined by their cytokines

Question 6

How does Th cell subset differentiation occur? (Think signals)

Answer 6

Following signal 1 and 2 control T cells activation
- signal 1 = TCR recognises Ag presented by MHC. gives antigen specificity, T cell receptor recognised antigen and knows its specific (doesn’t know where it comes from could just be from food)
signal 2 = APC upregulates B7 to signal to T cells that T cell that it comes from a microbe
- both these signals activates clonal expansion

Signal 3 - directs T cells effector function (signal 3 is given by cytokines and tells the T cells effector function the infection type and which subset to differentiate into.

Question 8

What happens once a T cell has differentiated?

Answer 8

Th1/2/17 cells migrate to infection site to aid innate immmune response

Th (Tfh) cells remain in LN to give persmision for B cells to activate

Question 9

How do B cells become activated to produce antibodies

Answer 9

• naive B cells don’t produce Ab
• membrane bound antibody serves as B cell antigen receptor
• once b cell detects antigen with its surface antigens and gets signal from T cell to activate, it secretes antibodies
• plasma cells = activated B cells that secrete antibodies

Question 10

What are the main effector mechanism of antibodies

Answer 10

• Opsonization: Antibodies coat microbes to help phagocytes take them up.
  • Complement activation: Triggers the complement pathway, aiding in lysis and opsonization.
  • Neutralization: Antibodies block pathogens/toxins from binding to host cells – the only mechanism antibodies do alone.
• lysis of microbes

Question 11

Describe the different effector classes of B cells

Answer 11

IgM, IgA, IgG, IgE
- function defined by class/isotype of antibody it produces
- diff classes interact w diff innate molecules to deal with diff infection types

Question 12

What is the complement system and what are its 3 main functions

Answer 12

Collection of proteins working together as an early warning system and to destroy pathogens
- recruitment of immune cells
- label microbes for phagocytosis by other cells (opsonisation)
- lyses pathogens

Question 13

Describe the 3 different pathways of the complement

Answer 13

Alternate, classical and lectin

Alternate & lectin
- faster bc they don’t need antibodies
- innate recognition

Classical
- uses Ab to identify pathogen
- Ab has high additive for Ag so it is a lot more effective and efficient due to higher affinity

Question 14

What is the shared outcome of different complement activation pathways?

Answer 14

• Both classical and lectin pathways activate C2, which cleaves and activates C4 (producing C4a).
• This leads to formation of C3 convertase, which cleaves C3.
• The product, C3b, binds to target cells and is the central effector molecule in all complement pathways.

Question 15

Describe complement mediated opsonisation

Answer 15

• binding of C3b to surface of microbe
• phagocytes have receptor called complement receptor that binds to C3b
• they are recognised by macrophages C3b receptor
  = phagocytosis and killing of microbe

Question 16

Describe complement mediated inflammation

Answer 16

During complement action, C3a, C4a and C5a are released which acts locally similar to inflammatory cytokines to recruit more immune cells to infection site and activates cells

Question 17

Describe complement mediated cytolysis

Answer 17

• MAC (membrane attack complex) forms in membrane and holes are punched in some pathogens
• water flows in, ions rich out and microbe bursts

Question 18

Describe functions of macrophage, neutrophils and dendritic cells

Answer 18

Macrophage ( jack of all trades)
- tissue resident
- recruited from blood monocytes

Neutrophils (killers on call)
- short lived
- recruited rapidly to infection site

Dendritic cell (talkers)
- specialise in activating naive T cells

Question 19

What are the main steps of phagocytosis

Answer 19

• phagocytes detect microbe via PRR, complement or Ab and extends pseudopodia to engulf microbe
• membrane invaginates forming an inside out vesicle called phagosome
• phagosome fuse w lysosome for form phagolysosom
• lysosome contains toxic molecules that degrade microbes

Question 20

How do phagocytes kill microbes inside the phagosome? (Phagosome Killing 1)

Answer 20

• Vacuolar ATPases acidify the phagosome by pumping in H+ ions.
• Phagocyte oxidase/NADPH oxidase initiate a respiratory burst, generating Reactive Oxygen Species (ROS) like H₂O₂.
• Neutrophils express myeloperoxidase (MPO), converting H₂O₂ into hypochlorite (HOCl, bleach).
• Proteolytic enzymes and elastase degrade microbial proteins and structures.
• Creates a highly toxic and degradative environment to destroy pathogens.

Question 21

How do macrophages contribute to phagosome killing? (Phagosome Killing 2)

Answer 21

• Macrophages produce nitric oxide (NO) via inducible nitric oxide synthase (iNOS).
• NO combines with superoxide to form peroxynitrite radicals – highly reactive and toxic.
• Phagosomes deprive pathogens of nutrients.
• Proteins like lactoferrin bind and remove iron, which microbes need to survive.
• Defensins: Small, directly microbicidal proteins that disrupt microbial membranes.

Question 22

How do antibodies make phagocytosis more efficient?

Answer 22

• Ab bind to microbe (opsonisation) with high specificity and affinity.
• Phagocyte binds to Ab via Fc Receptor (FcR)
• This is a more efficient detection system than PRRs

FCr binds to tail end of antibody (binding of opsonised microbes to phagocyte Fc receptors

Question 23

What are neutrophil extracellular traps?

Answer 23

• Neutrophil dies via a process called NETosis
• Nucleus swells and burst extruding DNA like a net.
• The DNA has anti-microbial molecules attached (e.g. defensins, proteases)
• Traps and kills bacteria, fungi, and viruses (pus)

Question 24

How do T cells enhance macrophage and neutrophil killing

Answer 24

Antibodies work as a bridge between innate and adaptive.
Th1 cells activates macrophages (respiratory burst enhanced)
Th17 produces IL-17 which recruits neutrophils and makes antimicrobial peptides, IL 22 p promotes barrier repair

Question 25

How do antibodies protect against infection and toxins through neutralization?

Answer 25

Without antibodies: • Microbes penetrate epithelial barriers and infect cells by binding to host receptors. • Toxins bind to cells, causing pathological effects (e.g., cell necrosis). With antibodies: • Antibodies bind microbes, forming a “net” to prevent entry through barriers. • Block microbial binding to host cell receptors, stopping infection. • Neutralize toxins by blocking their interaction with cells. Examples: • SARS-CoV-2 vaccines: Raise antibodies against Spike protein to block viral entry. • Tetanus vaccine: Produces antibodies that neutralize tetanus toxin. • Anti-venom: Contains antibodies (often from horses) that neutralize snake venom.

Question 26

What are granulocytes in host defence. Give 4 examples.

Answer 26

Prestore effector molecules in granules in cytoplasm ready to fire when activated. Mast cells, basophil, eosinophil, neutrophils

Question 27

What is Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)? How does ADCC work to eliminate infected or abnormal cells?

Answer 27

• Antibodies bind to antigens on the surface of a target cell or microbe. • The Fc region of the antibody binds to Fc receptors (FcγR) on innate immune cells, such as NK cells. • Multiple Fc-Ab interactions activate the effector cell. • The cell releases cytotoxic granules, killing the target. • This is a destructive process and can cause collateral damage. • IgG is key for ADCC; NK cells are especially important for killing infected intracellular cells.

Question 28

How does ADCC work against large extracellular parasites like helminths?

Answer 28

• Helminths are too large to be phagocytosed. • IgE antibodies bind to antigens on the helminth’s surface. • Eosinophils recognize IgE via FcεRI receptors. • Eosinophils become activated and degranulate, releasing toxic granules: • Major Basic Protein (MBP) • Eosinophil-Derived Neurotoxin (EDN) • Eosinophil Peroxidase (EPO) • Eosinophil Cationic Protein (ECP) • These substances are toxic to the parasite, but can also damage host tissue — response must be carefully regulated.

Question 29

What happens during mast cell degranulation in response to helminths or allergens?

Answer 29

• IgE binds to FcεRI receptors on mast cells. • Upon antigen binding and cross-linking, mast cells degranulate, releasing inflammatory mediators (e.g., histamine). • Weep response: Fluid leaks into the gut lumen, helping flush out helminths. • Sweep response: Histamine causes muscle contractions, pushing helminths out via faeces. • Cytokines amplify the inflammatory response. • This mechanism helps clear parasites without direct killing. • Drawback: Muscle contractions can cause wheezing and respiratory symptoms in allergic responses.

Question 30

What cytokines are produced by Th2 cells to enhance immunity to helminths?

Answer 30

- IL4 - tells B cell to recruit mast cells - IL5 - activates eosinophil - IL13 - increase mucus production and muscle contractions

Question 31

How do memory T and B cells enhance the immune response upon re-exposure?

Answer 31

• Memory T cells are already committed to the correct phenotype (e.g., Th1, Th2, Th17). • Memory B cells have undergone class switching and are committed to a specific antibody isotype (e.g., IgM, IgG, IgA, IgE). • Pathogen-specific antibodies are already present in the serum, ready to act. • Memory cells can rapidly amplify innate immune responses upon reinfection, providing faster and stronger protection.