Immunity against infection - bacteria

Question 1

What are the key features of innate defence mechanisms?

Answer 1

Rapid
Include barriers, complement (alternative pathway), phagocytes, NK cells, antimicrobial peptides
Act early as the first line of defence
Non-specific
Ineffective against many pathogens

Question 2

What are the features of acquired/adaptive defence mechanisms?

Answer 2

Involve antibodies and cell-mediated immunity
Take longer to develop
Exhibit memory
Enhance and focus innate defences
Less easily evaded by pathogens
Involve cross-talk with innate immunity

Question 3

What is the role of TH1 cells?

Answer 3

Active against intracellular pathogens
Activate macrophages
Stimulate cytotoxic T cells (CD8+)

Question 4

What is the role of TH2 cells?

Answer 4

Active against extracellular pathogens
Support antibody production (especially IgE class-switching)
Activate eosinophils, basophils, and mast cells

Question 5

What is the role of TH17 cells?

Answer 5

Active against extracellular bacteria and fungi
Attract inflammatory cells like neutrophils
Induced early in infection

Question 6

What are examples of Gram-positive bacteria?

Answer 6

Staphylococcus aureus
Streptococcus spp.

Question 7

What are examples of Gram-negative bacteria?

Answer 7

Campylobacter
Salmonella
Shigella
Haemophilus
Neisseria

Question 8

What components of bacterial cell walls can induce innate responses?

Answer 8

LPS (lipopolysaccharides)
Peptidoglycan

Question 9

How do bacterial components like LPS interact with the immune system?

Answer 9

Bind to Toll-like receptors (TLRs) on macrophages
Activate NOD-like receptors (intracellular sensors) in the cytoplasm

Question 10

What are Toll-like receptors (TLRs) and their effects?

Answer 10

Recognise pathogen-associated molecular patterns (PAMPs)
Promote inflammation
Stimulate dendritic cell maturation
Influence T cell differentiation
Activate B cells

Question 11

How do bacteria evade phagocytosis?

Answer 11

Some have protective capsules (e.g., Streptococcus pneumoniae)
Can be opsonised by antibody/complement for effective phagocytosis

Question 12

What roles do antibodies play in bacterial infections?

Answer 12

Opsonisation: Bind Fc receptors on phagocytes
Complement activation: Promote inflammation, opsonise, or lyse Gram-negative bacteria
Neutralise toxins (e.g., tetanus, diphtheria)
Prevent mucosal adherence by binding surface structures

Question 13

How do Gram-negative bacteria die via complement?

Answer 13

Killed by complement lysis (via the membrane attack complex, MAC)
Defects in terminal complement components increase susceptibility to Neisseria spp.

Question 14

How do some bacteria survive within phagocytes?

Answer 14

Mycobacterium tuberculosis inhibits lysosome-phagosome fusion

Question 15

What is the role of TH1 cells against intracellular bacteria?

Answer 15

Secrete cytokines like TNF-α and IFN-γ to activate macrophages
Enhance phagocytosis and antigen presentation

Question 16

What are the outcomes of responses to Mycobacterium leprae?

Answer 16

Tuberculoid leprosy: Strong TH1 response, slow progression, granuloma formation
Lepromatous leprosy: Strong TH2 response, disseminated infection, fatal. widespread infection due to ineffective macrophage activation by TH2 responses

Question 17

What protects against extracellular and intracellular bacterial infections?

Answer 17

Extracellular bacteria: Antibodies (e.g., against Streptococcus pneumoniae, Clostridium tetani)
Intracellular bacteria: T cell effector mechanisms (e.g., TH1 for Mycobacterium tuberculosis)

Question 18

What are the key differences between innate and adaptive immunity?

Answer 18

Innate Immunity: Rapid, non-specific, includes barriers, complement, phagocytes, NK cells, antimicrobial peptides.
Adaptive Immunity: Slower to develop, specific, includes antibodies and cell-mediated immunity, exhibits memory.

Question 19

How do CD8+ T cells contribute to bacterial defence?

Answer 19

Kill infected cells displaying bacterial antigens via MHC class I.
Release perforins and granzymes to induce apoptosis in infected cells.

Question 20

: What are immune-privileged sites, and why are they important?

Answer 20

Sites like the eye, testis, and CNS have barriers that limit immune responses.
Reduce inflammation and damage in sensitive areas but can allow persistent infections.

Question 21

Why are TH17 cells critical in immune defence?

Answer 21

Recruit neutrophils early in infection.
Effective against extracellular bacteria and fungi.
Secrete IL-17 to promote inflammation.

Question 22

How do antibodies neutralise bacterial toxins?

Answer 22

Bind directly to toxins, preventing them from interacting with host cells.
Examples: Tetanus and diphtheria toxins.

Question 23

What is the vaccine for Streptococcus pneumoniae?

Answer 23

Composed of 23 polysaccharide serotypes.
Conjugate vaccine available to provide broader and longer-lasting immunity.

Question 24

How does Mycobacterium tuberculosis evade immune defences?

Answer 24

Inhibits lysosome-phagosome fusion within macrophages.
Survives and replicates inside phagocytes.

Question 25

What are the effects of complement activation against bacteria?

Answer 25

Promotes inflammation via C3a, C5a.
Opsonisation via C3b binding.
Direct lysis of Gram-negative bacteria through the Membrane Attack Complex (MAC).

Question 26

How do different antibodies contribute to bacterial defence?

Answer 26

IgG: Opsonises bacteria for phagocytosis.
IgA: Prevents bacterial adherence to mucosal surfaces.
IgE: Involved in responses to parasites and allergies but supports TH2 against extracellular pathogens.