Human Defence Systems

This lesson covers the body's non-specific defence systems as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the physical and chemical barriers that form the first line of defence against pathogens, including the skin, mucous membranes, cilia, stomach acid and other mechanisms.

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Lines of Defence

The human body has a multi-layered defence system to prevent pathogens from entering and causing disease. These defences are divided into two main categories:

1. Non-specific defences (first line of defence) — physical and chemical barriers that protect against ALL pathogens. They do not target specific pathogens.
2. Specific defences (the immune response) — targeted responses by white blood cells against particular pathogens. This is covered in the next lesson.

graph TD
    A[Pathogen attempts<br/>to enter body] --> B{First line of defence<br/>Non-specific barriers}
    B -->|Pathogen blocked| C[No infection]
    B -->|Pathogen gets through| D{Second line of defence<br/>Immune response}
    D -->|Pathogen destroyed| E[Recovery]
    D -->|Pathogen not controlled| F[Disease develops]

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The Skin

The skin is the body's largest organ and the most important physical barrier against infection.

How the Skin Protects