C3.2 Defence Against Disease

What are pathogens?

Disease-causing agents: bacteria, viruses, fungi, protists, prions (not archaea).

How do pathogens cause disease through mechanisms?

Invasion followed by multiplication leads to damage via toxins, cell lysis, and inflammation.

What are the primary transmission routes for pathogens?

Airborne, fecal-oral, vector, body fluids, and fomites.

What analogy describes a pathogen's interaction with a host cell?

A pathogen is like a hacker that hijacks the host cell computer.

Compare the speed and specificity of innate and adaptive immunity?

Innate: immediate, non-specific, no memory. Adaptive: slower on first exposure, antigen-specific, builds memory.

How long do innate vs. adaptive responses typically take?

Innate takes hours; adaptive takes days but is faster upon re-exposure.

How does skin act as a primary defense?

It has a keratinized outer layer, acidic pH, sebum with antimicrobial substances, and undergoes constant shedding.

How do mucous membranes defend against infection?

They secrete sticky mucus with lysozyme to trap pathogens and use cilia to sweep debris.

What are "other" barriers to infection besides skin and mucus?

Stomach acid and normal microbiota that compete with invaders.

What is the role of platelets in blood clotting?

They stick to damaged vessels to form a temporary plug.

What is the cascade of clotting factors during blood clotting?

Prothrombin → thrombin → fibrinogen → fibrin.

What is the function of the fibrin mesh?

It traps cells to form a clot that hardens into a scab, preventing blood loss and blocking pathogen entry.

What are the three essential components of the clotting process?

Platelets, clotting factors, and fibrin mesh.

What is an antigen?

A non-self molecule that triggers an immune response.

Where can antigens be found?

On pathogens, transplants, tumor cells, or red blood cells.

How are antigens recognized to trigger clonal selection?

They are recognized by B-cell receptors.

Why should you mention antigens when discussing transplant rejection?

Antigens on the transplant tissue are what trigger the immune system to reject the organ.

Which cells produce and secrete antibodies?

B cells (specifically plasma cells) produce them when activated.

What is the role of Helper T cells in immunity?

They release cytokines that stimulate B cells.

What is the function of Cytotoxic T cells?

They kill infected cells.

How do antibodies help defeat pathogens?

They neutralize pathogens, block toxins, or tag them for destruction.

What must be mentioned when explaining antibody production?

The role of Helper T cells in stimulating the process.

What happens during the secondary immune response?

On re-exposure, the response is faster, stronger, and longer-lasting due to memory cells.

What is the basis for long-term immunity and vaccination?

The persistence of memory B and T cells after an initial infection or vaccination.

Do vaccines cause disease?

No, they create memory cells without causing the actual disease.

How is HIV transmitted?

Through blood, semen, vaginal/rectal secretions, and breast milk.

What are common ways HIV is NOT spread?

Casual contact, air, or insect bites (like mosquitoes).

Which cells and receptors does HIV target?

HIV is a retrovirus that binds to CD4 receptors on helper T cells.

How does HIV replicate inside a host cell?

It uses reverse transcriptase to convert RNA into DNA, which then integrates into the host genome.

What is the difference between HIV and AIDS?

HIV is the virus; AIDS is the syndrome of immune collapse caused by the destruction of T cells.

How do antibiotics work against bacteria?

They disrupt cell wall synthesis, 70S ribosomes, DNA replication, or folate pathways.

Why are antibiotics ineffective against viruses?

Viruses do not have their own metabolism or ribosomes for the drug to target.

How does antibiotic resistance evolve?

Through mutation and natural selection, where resistant bacteria survive, spread, and often share genes via plasmids.

What are two examples of antibiotic-resistant bacteria?

MRSA and resistant TB.

What is a zoonosis?

A disease that jumps from animals to humans.

Give a named example of a zoonotic disease and its source?

Rabies (dogs/bats), TB (cattle), Japanese encephalitis (pigs/birds), or COVID-19 (bats).

How did pasteurization affect disease transmission?

It reduced the transmission of bovine TB to humans.

What do vaccines contain?

Antigens or nucleic acids used to prime adaptive immunity.

What are the different types of vaccines?

Live attenuated, inactivated, subunit, and mRNA/DNA.

Do vaccines cure diseases?

No, vaccines prevent disease; they do not cure it.

What is herd immunity?

When a majority of a population is immune, reducing disease spread to protect vulnerable groups.

What does the threshold for herd immunity depend on?

The R0 (basic reproduction number) of the disease.

What are the herd immunity thresholds for Measles and Polio?

Measles: ~95%; Polio: ~80-85%.

What data is included in pandemic evaluation?

Infection rates, mortality, and vaccination coverage.

What mathematical tools are used to evaluate COVID-19 data?

Percentage change (over time) and percentage difference (between groups).

What contextual factors must be considered when interpreting pandemic data?

Healthcare systems, population density, and socioeconomic factors