Pathogens, Immunity and Vaccination

GCSE Biology · Infection and Response

Pathogens

Microorganisms that cause communicable (infectious) disease:

Bacteria – reproduce rapidly, produce toxins (e.g. Salmonella).
Viruses – reproduce inside cells, damaging them (e.g. measles, HIV).
Fungi – e.g. rose black spot.
Protists – often spread by a vector (e.g. malaria by mosquitoes).

The body's defences

Barriers: skin, mucus, cilia, stomach acid.
White blood cells:
Phagocytosis – engulf and digest pathogens.
Antibodies – lock onto specific antigens.
Antitoxins – neutralise toxins.

Immunity & memory

Antibodies are specific to one pathogen's antigens. After infection, memory cells remain, giving a faster, larger response next time → immunity.

Vaccination

Inject dead or weakened pathogens (or antigens) → the body makes antibodies and memory cells without illness, so you're protected if exposed later.

Herd immunity: if enough people are vaccinated, the disease can't spread easily.

Antibiotics

Kill bacteria (not viruses). Overuse leads to resistant strains (e.g. MRSA), so they must be used carefully.

Exam tip

Vaccines expose the body to antigens, producing memory cells — link these terms. Antibiotics do not work on viruses.