Human defence systems: skin

The idea

The skin is the body's first line of defence against pathogens. Its outer layer of dead, keratinised cells forms a tough physical barrier that most bacteria, viruses and fungi cannot penetrate. Sebaceous glands secrete sebum, an oily substance that keeps skin supple and lowers surface pH, inhibiting microbial growth. When skin is cut, blood clots seal the wound, preventing entry of microorganisms. Sweat also contains lysozyme, an enzyme that breaks down bacterial cell walls. Intact skin therefore acts as a non-specific defence, meaning it works against all pathogens in the same way, unlike antibodies which target specific antigens. Damage such as a burn or graze immediately increases infection risk, showing the barrier's importance.

What earns the marks

• Skin is a physical barrier that prevents pathogens entering the body.
• The outer layer consists of dead cells that pathogens cannot easily penetrate.
• Sebum from sebaceous glands lowers skin pH and inhibits microorganism growth.
• Sweat contains lysozyme, which destroys bacterial cell walls.
• Blood clotting at a break in the skin seals the wound and blocks pathogen entry.
• Skin defence is non-specific because it acts against all pathogens in the same way.

Common mistakes and how to put them right

• Saying skin kills all pathogens.

  Put it right by explaining it mainly blocks entry, with sebum and lysozyme inhibiting or destroying some microbes.

• Confusing sebum with sweat.

  Put it right: State that sebum is an oily secretion from sebaceous glands, while sweat is watery and contains lysozyme.

• Describing skin as a specific immune response.

  Put it right by stating it is a non-specific first-line barrier, not antibody production.

Exam tips

• Link each skin feature to its defensive function rather than listing features alone.
• Use the term non-specific defence when contrasting skin with antibody action.
• If asked about a cut or burn, explain how the broken barrier allows pathogens to enter.