The Immune System

The immune system is the body's defence against infectious disease caused by pathogens — organisms or agents that cause disease, including bacteria, viruses, fungi, and protoctists. Understanding how the body recognises and destroys pathogens, the distinction between non-specific and specific immunity, and the principles of vaccination and immunological therapies is essential for A-Level Biology.

Key Definition: A pathogen is an organism or agent that causes disease. Pathogens include bacteria, viruses, fungi, and protoctists. They cause disease by damaging host cells and tissues, and/or by producing toxins.

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Non-Specific Defences (Innate Immunity)

Non-specific defences are the body's first and second lines of defence. They act against all pathogens, regardless of type, and do not require prior exposure. They respond rapidly but do not confer lasting immunity.

First Line of Defence — Physical and Chemical Barriers