CB5.2 — Body defences (Edexcel 1SC0)

Physical and chemical barriers (non-specific)

• Skin: physical barrier; prevents pathogen entry.
• Mucus: traps pathogens in the nose and airways.
• Cilia: sweep mucus (containing trapped pathogens) away from lungs.
• Stomach acid: kills most pathogens that are swallowed.
• Tears and saliva: contain lysozyme enzyme that breaks down bacterial cell walls.

The immune system (specific)

White blood cells (lymphocytes):

1. Pathogen enters → lymphocytes recognise foreign antigens on the pathogen's surface.
2. Lymphocytes produce specific antibodies that bind to those antigens (lock-and-key fit).
3. Antibodies: label pathogens for destruction; neutralise toxins; clump bacteria (agglutination).
4. Memory cells are produced → faster, stronger response on second exposure (immunological memory).

Phagocytes:

• Engulf and digest pathogens by phagocytosis (non-specific).

Vaccination

A vaccine contains dead, weakened, or antigen-only forms of a pathogen. It stimulates the immune system to produce antibodies and memory cells without causing the disease.

Herd immunity: if enough of the population is vaccinated, the pathogen cannot spread — even unvaccinated individuals are protected.

Active immunity: producing antibodies in response to infection or vaccination (long-lasting). Passive immunity: receiving antibodies from another source (e.g. breast milk, injection) — short-lived.