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IB DemystifiedMYP Sciences
Pathogens, disease and immunity
Every day your body meets millions of microorganisms, and almost none of them make you ill. Layers of defence, from skin to antibodies to memory cells, keep them out or destroy them, and vaccines train those defences before the real threat arrives.
Recommended for MYP 4 · eAssessment priority · About 3 lessons · Criteria A, B, C and D
Figure 1. A second exposure to the same pathogen gives a much faster, larger antibody response.
describe the main types of pathogen and how they spread
explain the body's barriers to infection
explain how phagocytes and lymphocytes destroy pathogens
explain immunity and vaccination using primary and secondary responses
explain how antibiotics work and how resistance develops
evaluate public-health decisions about vaccines and antibiotics
Before you start
You will use these skills. If any feel shaky, review them first.
cell structure, including bacterial cells (see Cells and microscopy)
the blood and circulatory system
natural selection (see Evolution)
Key vocabulary
Pathogen
A microorganism that causes disease: a bacterium, virus, fungus or protist.
Antigen
A molecule, usually on a pathogen's surface, that the immune system recognises as foreign.
Antibody
A protein made by lymphocytes that binds to one specific antigen.
Phagocyte
A white blood cell that engulfs and digests pathogens.
Lymphocyte
A white blood cell that makes antibodies; some become memory cells.
Memory cell
A long-lived lymphocyte that recognises a pathogen met before and triggers a fast secondary response.
Vaccine
A preparation of dead or weakened pathogens, or their antigens, that produces immunity without disease.
Herd immunity
Protection of unimmunised people because most of the population is immune.
Understanding immunity
What is it?
Communicable diseases are caused by pathogens that spread between people, through droplets, contaminated food and water, direct contact or animal vectors. Immunity is the body's ability to recognise and destroy a particular pathogen quickly.
Why does it work?
Barriers (skin, mucus and cilia, stomach acid) stop most pathogens entering. Those that get in are engulfed by phagocytes. Lymphocytes whose antibodies fit the pathogen's antigens multiply and release antibodies; some become memory cells. The first response is slow, so we may become ill, but memory cells make any later response fast and large enough to stop symptoms.
How do we know?
In 1796 Edward Jenner showed that exposure to mild cowpox protected people against smallpox. Measuring antibodies in blood after vaccination shows the primary and secondary responses directly, and large trials compare disease rates in vaccinated and unvaccinated groups.
Why does it matter?
Vaccination eradicated smallpox, declared in 1980, and has greatly reduced diseases such as polio and measles. Antibiotics turned once-deadly infections into treatable ones, but resistance now threatens that progress.
What does it connect to?
Immunity links to cells, blood and natural selection in biology; to the chemistry of medicines and disinfectants; and to statistics and ethics in public-health decisions.
Defences, vaccines and medicines
Barriers: the skin; mucus and cilia in the airways; acid in the stomach; enzymes in tears.
Phagocytes: engulf and digest any pathogen (non-specific).
Lymphocytes: make antibodies that fit one antigen (specific) and form memory cells.
Vaccines: give a primary response and memory cells without illness; high coverage gives herd immunity.
Antibiotics: kill bacteria or stop them growing, but have no effect on viruses. Overuse selects for resistant bacteria.
Antibodies fall after an infection (Figure 1), but immunity lasts because memory cells remain.
Disease in the real world
In recent years, Pakistan and Afghanistan have been the only countries where wild poliovirus has kept circulating, and vaccination teams work to reach every child. Hospitals track antibiotic-resistant infections such as MRSA and limit antibiotic use to slow resistance. Simple measures, including handwashing, clean water and mosquito nets, still prevent millions of infections every year.
Worked examples
Example 1: explaining immunity
Explain why a child who has had chickenpox is unlikely to catch it again.
During the first infection, lymphocytes made antibodies against the chickenpox antigens, and memory cells were formed.
If the virus enters again, memory cells recognise its antigens quickly.
A fast, large secondary response destroys the virus before it causes symptoms.
Example 2: resistance as natural selection
Explain why MRSA spreads in hospitals where antibiotics are used heavily.
A mutation gives some bacteria resistance to an antibiotic.
Heavy use kills non-resistant bacteria, leaving the resistant ones with less competition.
They reproduce and spread between patients, so resistant strains become common.
In the eAssessment
Health questions often combine a graph, a lab dataset and a real-world decision. Expect:
Interpret antibody graphs: compare primary and secondary responses, and read peaks and times.
Process lab data: means, percentages and areas of clear zones.
Explain sequences: vaccination, immunity and resistance in clear, ordered steps.
Discuss: vaccination policy and antibiotic use, weighing health, freedom and economics.
Common ways to lose marks: saying antibiotics treat viral infections; confusing antigens with antibodies; saying people (rather than bacteria) become resistant; and saying immunity is due to antibodies staying in the blood.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Show
Investigation: modelling the spread of a disease
Partially guided investigation · about 40 minutes · whole class
Research question
How does the number of contacts each person makes (1, 2 or 3 exchanges) affect how many people in a class of 25 become "infected" in a simulated outbreak?
Scientific background
Communicable diseases spread through contact between people. In this model, one secret "infected" cup contains dilute sodium hydroxide; the others contain water. Exchanging liquid spreads the "infection", which an indicator reveals at the end.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain why each control matters.
Apparatus
Labelled plastic cups each with 20 cm³ of water (one secretly with 0.1 mol/dm³ sodium hydroxide), dropping pipettes, phenolphthalein indicator, record sheet.
Method
Each student takes a cup. In round 1, pair up with one person and pour all the liquid from one cup into the other and back again.
Record who you exchanged with.
Your teacher adds indicator to every cup; pink means "infected".
Repeat with fresh cups for 2 and 3 exchanges (with different partners), and use the contact records to try to trace the first case.
Safety. Sodium hydroxide, even when dilute, is an irritant: wear eye protection, do not drink from cups, and wash any splashes off skin with plenty of water. Wipe up spills immediately.
Then evaluate: how well does this model represent a real outbreak, and what does it leave out (such as immunity or recovery)?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
