Pathogens and transmission: fixed definitions
A pathogen is a disease-causing organism. A transmissible disease is one in which the pathogen can be passed from one host to another. Transmission is examined as two routes you must be able to classify: direct contact (e.g. through blood or other body fluids) and indirect (e.g. from contaminated surfaces, food, water or air). Questions give a scenario ('a child drinks water containing sewage') and ask the route. Answer 'indirect, through contaminated water'. The body's first-line defences against pathogens entering are a standing list: the skin as a barrier, mucus and cilia in the airways trapping and moving pathogens, hydrochloric acid in the stomach killing pathogens in food.
Phagocytes vs lymphocytes: never confuse these
Both are white blood cells, but the exam pays for the right one:
| Cell | What it does |
|---|---|
| Phagocyte | Engulfs and digests pathogens (phagocytosis); acts on any pathogen, non-specific |
| Lymphocyte | Produces antibodies specific to a particular pathogen's antigens |
An antigen is a molecule (often on the surface of a pathogen) that the immune system recognises as foreign. An antibody is a protein, made by lymphocytes, that is specific (complementary) to a particular antigen. It binds to the antigen, marking the pathogen for destruction or causing pathogens to clump together. The specificity point is the Extended mark: each antibody fits one antigen, the way an enzyme fits one substrate.
Active vs passive immunity
Active immunity is defence gained when an antigen enters the body and the person's own lymphocytes make antibodies. It is long-lasting because memory cells remain. It can be natural (after infection) or artificial (after vaccination). Passive immunity is short-term protection from antibodies introduced from outside, e.g. across the placenta or in breast milk, with no memory cells made, so it does not last. The exam contrast question wants: active = you make the antibodies, memory cells, long-term; passive = antibodies given to you, no memory cells, short-term.
Vaccination: the chain that scores every mark
A vaccine contains weakened or dead pathogens, or their antigens. The mechanism is a fixed chain. Write all five links:
- The vaccine puts antigens into the body.
- Lymphocytes recognise the antigens and produce specific antibodies.
- Memory cells are also produced and remain in the body.
- If the real pathogen later infects the person, the memory cells respond faster and produce more antibodies more quickly.
- The pathogen is destroyed before it can cause symptoms. The person is immune.
The 'herd immunity' extension: if enough of a population is vaccinated, transmission between people is reduced, protecting even the unvaccinated. Examiners reserve a mark for stating that this breaks the chain of transmission, not just that 'fewer people get ill'.