(3) How do our bodies defend themselves against infectious diseases?

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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points and more examples about the body's chemical systems for defending itself against pathogens

Notes for biology students on chemical defences in the human body and why they’re so important in understanding immunity and disease prevention:

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Chemical Defences Against Pathogens

While physical barriers aim to block pathogens from entering the body, chemical defences destroy or neutralise them once they’ve breached—or to reinforce physical defences.

These are non-specific defence mechanisms, meaning they’re not tailored to a particular pathogen but instead protect against a wide range of microbes.

1. Lysozymes (in Tears, Saliva, and Mucus)

• Function: Lysozymes are enzymes that attack bacterial cell walls.

• Where found:

  • Tears (lachrymal glands)

  • Saliva (salivary glands)

  • Nasal mucus and other mucosal secretions

• Action: They hydrolyse the peptidoglycan in bacterial cell walls, causing them to rupture.

2. Hydrochloric Acid in the Stomach

• Extremely low pH (~1–2) kills most pathogens ingested through food and drink.

• Destroys microbes that may enter with swallowed mucus from the respiratory tract.

• Not specific to any one pathogen—effective against most bacteria and viruses.

3. Sebum (Oily Secretion from Skin)

• Besides being a physical barrier, sebum also has antimicrobial properties.

• It helps lower skin pH, creating an environment hostile to many pathogens.

4. Vaginal Secretions

• Have an acidic pH due to lactic acid produced by natural flora.

• This acidity inhibits the growth of pathogens, providing protection.

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Why These Defences Matter for Human Biology and Exams

Understanding Immunity

• Builds the foundation before you study specific immunity (like lymphocytes and antibodies).

• Highlights the body’s immediate, broad-spectrum defence tactics.

Common GCSE Question Topic

• Often part of “describe how the body defends itself non-specifically” type questions.

• May be tested alongside physical barriers in multiple-choice or extended-answer formats.

Link to Health and Disease

• Understanding these defences helps explain:

  • Why hand-to-mouth transmission is risky.

  • Why stomach acid reducers (like antacids) might make someone more vulnerable.

  • How disrupting these secretions can lead to secondary infections.

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Learning objectives for this section on our body's chemical defences against pathogens

Be able to describe chemical ways in which the body defends itself against pathogens.

Know that lysozyme in tears protects against pathogens potentially infecting our eyes.

Know about the action of the strong acid, hydrochloric acid in our stomach that kills bacteria.

Know that our skin has sebaceous glands that secrete antimicrobial molecules to kill bacteria.

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