Immunity and vaccination
Antigens and antibodies (Supplement)
- Every pathogen carries antigens on its surface, each with its own shape.
- Antibodies are proteins made by lymphocytes.
- An antibody's shape is complementary to one antigen — it fits only that pathogen (like an enzyme and its substrate).
Practice
An antibody fits only one pathogen because:
Each antibody has a shape complementary to one antigen, so it binds only that pathogen.
Active immunity and vaccination
- Active immunity = protection from your own body making antibodies (after infection or vaccination).
- A vaccine works like this:
- weakened pathogens or just their antigens are put in — they cannot make you ill,
- your lymphocytes make antibodies,
- the body keeps memory cells for years.
- If the real pathogen arrives later, memory cells make antibodies fast, so you don't get ill.
Practice
Vaccination gives long-term protection because the body makes:
Vaccination triggers antibody production AND memory cells, which give a fast response later.
Passive immunity
- Passive immunity = protection from antibodies made by another body.
- A baby gets antibodies from its mother across the placenta and in breast milk.
- It is short-term because no memory cells are made.
Practice
Passive immunity (e.g. from breast milk) is short-term because:
The antibodies come from another body and no memory cells form, so protection fades.
Cholera (Supplement)
- Cholera is caused by a bacterium that spreads in contaminated water.
- Its toxin makes the small intestine pump chloride ions in.
- Water then follows by osmosis, causing diarrhoea and dehydration.
Practice
Put the cholera events in order.
Toxin → chloride ions in → water in by osmosis → diarrhoea and dehydration.
You've got it
Key idea
- an antibody's shape is complementary to one antigen (Supplement)
- active immunity (infection/vaccination) makes memory cells → long-term; passive (placenta, breast milk) has no memory cells → short-term
- cholera toxin → chloride ions into the gut → water in by osmosis → diarrhoea + dehydration