Immunology Exam: Topics 3 and 4

What is a pathogen?

A microorganism that causes disease.

What is an opportunistic pathogen?

A bacterium that usually does not cause disease but can if immunity is weakened or it enters the wrong body site.

What is a virulence factor?

A bacterial feature that helps cause disease, such as adhesins, capsules, toxins, or invasion proteins.

What are the 5 steps of bacterial pathogenesis?

Entry → colonisation → evasion of host defences → multiplication/dissemination → host damage.

What is normal microbiota?

Microbes that normally live on body surfaces and help protect against pathogens by competition.

Why are normally sterile sites important?

Blood, CSF, lymph, and muscles should not contain bacteria; bacteria there usually indicate infection.

How can new bacterial pathogens emerge?

Mutation, horizontal gene transfer, zoonotic transfer, new virulence genes, hospitals, travel, and food spread.

What is a zoonotic disease?

A disease transmitted from animals to humans.

What are Koch’s postulates used for?

To prove that a specific microbe causes a specific disease.

What are the main steps of Koch’s postulates?

Find microbe in diseased host → isolate in pure culture → cause disease in new host → re-isolate same microbe.

What is a limitation of Koch’s postulates?

They do not work well for obligate intracellular pathogens and often require animal models.

What are physical barriers?

First-line defences such as skin, stomach acid, mucus, cilia, peristalsis, and lysozyme.

What are innate immune defences?

Complement, phagocytes, inflammation, TLR signalling, and cytokines.

What is colonisation?

Establishment of bacteria in the host.

Why is adherence important?

It prevents bacteria from being washed away and allows colonisation.

What are adhesins?

Bacterial surface molecules that attach to host cells.

What are pili/fimbriae?

Hair-like structures made of pilin that allow loose attachment to host cells.

What are afimbrial adhesins?

Surface proteins that allow close/intimate attachment to host cells.

What does a capsule do?

Helps bacteria avoid phagocytosis, resist drying, and adhere to tissues.

What is a biofilm?

A bacterial community attached to a surface and protected by extracellular slime/DNA.

Why are biofilms hard to treat?

They protect bacteria from antibiotics and immune cells.

Why do bacteria invade host cells?

To hide from immunity, escape microbiota, survive intracellularly, or spread deeper into tissues.

What is the zipper mechanism?

Tight binding between bacterial invasin and host receptor causes the host membrane to wrap around the bacterium.

Example of zipper mechanism?

Listeria monocytogenes uses internalin to bind cadherin.

What is the trigger mechanism?

Bacteria inject effectors using T3SS, causing membrane ruffling and uptake.

Example of trigger mechanism?

Salmonella and Shigella.

How do bacteria damage the host?

Directly by toxins or indirectly by immune overreaction/immunopathology.

What is an exotoxin?

A secreted bacterial toxin, usually a protein.

Why can exotoxins be useful for vaccines?

They can often be turned into toxoids.

What are the 3 main exotoxin types?

A-B toxins, membrane-disrupting toxins, and superantigens.

What is an A-B toxin?

A toxin with a binding B subunit and active A subunit.

What does the B subunit do?

Binds to specific host cell receptors.

What does the A subunit do?

Enters the cell and causes damage.

What is diphtheria toxin?

A simple AB toxin that inhibits protein synthesis and damages tissues.

What causes diphtheria?

Corynebacterium diphtheriae.

What is botulinum toxin?

A neurotoxin from Clostridium botulinum that causes flaccid paralysis.

How does botulinum toxin affect muscles?

It prevents muscle contraction.

What is cholera toxin?

An AB5 enterotoxin from Vibrio cholerae.

How does cholera toxin cause disease?

Increases cAMP, causing massive water/electrolyte loss and rice-water stools.

What are membrane-disrupting toxins?

Toxins that damage host cell membranes.

What does α-haemolysin do?

Forms pores in host membranes, causing water entry and cell lysis.

What does Clostridium perfringens α-toxin do?

Acts as phospholipase C, damaging membranes and causing tissue necrosis.

What are superantigens?

Toxins that overactivate T cells by linking MHC II and TCR non-specifically.

What can superantigens cause?

Massive cytokine release, shock, organ failure.

What is endotoxin?

A structural toxin, not secreted, found in Gram-negative bacteria.

What is the main endotoxin?

LPS, especially lipid A.

What does LPS cause?

Fever, inflammation, IL-1/TNF release, and possible shock.

What is direct transmission?

Spread by direct contact, including sexual contact or mother-to-child.

What causes syphilis?

Treponema pallidum.

What is vertical transmission?

Transmission from mother to unborn child or newborn.

What causes Group B Streptococcal disease?

Streptococcus agalactiae.

Why is Group B Strep important?

It can cause neonatal sepsis, meningitis, and pneumonia.

What is airborne transmission?

Spread through droplets or dust in air.

What causes tuberculosis?

Mycobacterium tuberculosis.

Why is TB hard to clear?

It survives inside macrophages and resists killing mechanisms.

What is a vector?

A living transmitter of disease, such as fleas, ticks, or mosquitoes.

What causes plague?

Yersinia pestis.

How can food/water transmit bacteria?

Poor hygiene, undercooking, poor storage, contaminated water, or unpasteurised foods.

Why is Typhoid Mary important?

She showed that healthy asymptomatic carriers can spread disease.

What is an antibiotic?

A substance that kills bacteria or inhibits their growth.

What does bactericidal mean?

Kills bacteria.

What does bacteriostatic mean?

Stops bacterial growth without directly killing them.

What is selective toxicity?

A drug harms bacteria more than host cells.

Why are antibiotics selectively toxic?

They target bacterial features such as peptidoglycan cell walls or 70S ribosomes.

What is a broad-spectrum antibiotic?

Works against many types of bacteria.

What is a narrow-spectrum antibiotic?

Works against a limited group of bacteria.

What is MIC?

Minimum inhibitory concentration: lowest drug concentration that prevents growth.

What is MLC?

Minimum lethal concentration: lowest drug concentration that kills bacteria.

What is disk diffusion testing?

Antibiotic discs are placed on bacteria-covered agar to see zones of inhibition.

What is the Kirby-Bauer test?

A commonly used disk diffusion antibiotic susceptibility test.

What is an E-test?

A strip with an antibiotic gradient used to estimate MIC.

What are the main antibiotic targets?

Cell wall synthesis, protein synthesis, metabolism, nucleic acid synthesis, and membranes.

What do penicillins target?

Peptidoglycan cell wall synthesis.

How do penicillins work?

They bind PBPs and block transpeptidation/cross-linking of peptidoglycan.

Why do penicillins work best on growing bacteria?

Growing bacteria are actively making new peptidoglycan.

What is the β-lactam ring?

The essential active structure in penicillin.

What does β-lactamase do?

Breaks the β-lactam ring and inactivates penicillin.

What are PBPs?

Penicillin-binding proteins involved in peptidoglycan cross-linking.

What do aminoglycosides target?

The 30S ribosomal subunit.

How do aminoglycosides work?

Cause misreading of mRNA and inhibit protein synthesis.

Example of an aminoglycoside?

Streptomycin.

How can bacteria resist aminoglycosides?

Efflux, reduced entry, ribosomal mutation, or enzymatic modification.

What are sulfonamides?

Metabolic antagonists that block folic acid synthesis.

What molecule do sulfonamides mimic?

PABA.

Why do sulfonamides stop bacterial growth?

They prevent folate production, which is needed for DNA/RNA building blocks.

Are sulfonamides bacteriostatic or bactericidal?

Bacteriostatic.

What do fluoroquinolones target?

DNA gyrase and topoisomerase IV.

How do fluoroquinolones work?

They block DNA coiling during replication.

Are fluoroquinolones bactericidal?

Yes.

What do polymyxins target?

Bacterial membranes, especially Gram-negative outer membranes.

How do polymyxins work?

They act like detergents and disrupt the membrane.

What are the 4 main resistance mechanisms?

Modify target, inactivate drug, reduce drug concentration, or bypass inhibited pathway.

How do plasmids contribute to resistance?

They can carry resistance genes and spread between bacteria.

What is efflux?

Pumping antibiotics out of the bacterial cell.

Why can reduced permeability cause resistance?

Less antibiotic enters the bacterial cell.

Why is antibiotic resistance a major problem?

Resistant bacteria survive treatment and can spread resistance genes.

What is intracellular trafficking?

The controlled movement of proteins, lipids, vesicles, and other molecules inside a cell. It allows proteins to reach the correct organelle, membrane, or extracellular space.

Why is intracellular trafficking important in microbiology and immunology?

Pathogens, especially viruses, exploit host trafficking pathways to enter, replicate, move through, and exit cells. Immune molecules such as antibodies and cytokines also rely on trafficking for secretion.

What is the secretory pathway?

A pathway that moves proteins from the rough ER → Golgi apparatus → final destination, such as plasma membrane, lysosome, secretory vesicle, or outside the cell.

What is a signal sequence or motif?

A short amino acid sequence that acts like a “molecular address”, directing a protein to a specific location in the cell.