Disease and the immune system

Pathogens

A pathogen is a microorganism that causes disease.

First line of defence — non-specific

• Skin acts as a physical barrier.
• Mucus in the airways traps particles; cilia waft them up to be swallowed.
• Stomach acid (HCl) kills swallowed pathogens.
• Tears contain lysozyme.

Second line — the immune response (specific)

White blood cells respond in three ways:

1. Phagocytosis — the white cell engulfs and digests the pathogen.
2. Antibody production — lymphocytes recognise specific antigens on the pathogen surface and produce complementary antibodies that lock onto them, marking them for destruction.
3. Antitoxins — neutralise toxins released by some bacteria.

Memory cells remain after the first infection, giving fast immunity if you meet the same pathogen again.

Vaccination

A vaccine contains a dead, weakened or fragment form of the pathogen. The body produces antibodies and memory cells without becoming ill. If the real pathogen later enters the body, memory cells produce antibodies very quickly, before symptoms develop.

Herd immunity — when enough of a population is vaccinated, the pathogen cannot spread, protecting people who cannot be vaccinated.

Antibiotics vs antivirals

Antibiotics (e.g. penicillin) kill bacteria. They do not affect viruses (viruses use the host's own cells, so few targets exist). Overuse breeds resistant bacteria such as MRSA.

Monoclonal antibodies (Higher)

Identical antibodies produced from a single B-cell clone. Used in pregnancy testing (target hCG), in diagnosis (locate cancer cells), and in some cancer treatments (deliver a drug only to tumour cells).

CCEA tip

When asked "why don't antibiotics work on a cold?", you need both halves: "a cold is caused by a virus" AND "antibiotics only kill bacteria".