The Immune System

The immune system is the body's specific defence against pathogens. While non-specific defences (skin, mucus, phagocytes) work against all pathogens, the immune system targets individual pathogens using specialised white blood cells called lymphocytes. This lesson explains how lymphocytes recognise and destroy pathogens, and how immunological memory provides long-term protection.

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Antigens — The Key to Specificity

Every pathogen has unique molecules on its surface called antigens. These antigens are like molecular fingerprints — they are different for every type of pathogen.

Term	Definition
Antigen	A unique protein on the surface of a pathogen that the immune system can recognise
Self antigen	A protein on the surface of your own cells — your immune system recognises these as "self" and does not attack them
Non-self antigen	A protein on the surface of a foreign cell or pathogen — triggers an immune response

The ability to distinguish between self and non-self is the foundation of the immune system. When the body detects non-self antigens, it knows a pathogen has entered and mounts a specific immune response.