Topic 3.4

First Line of Defence: Physical and Chemical Barriers

• This is the outermost layer of protection, acting as a wall to stop invaders from entering your body. Think of your skin, mucus, and tears as physical barriers, while stomach acid and enzymes in saliva work as chemical defenders. Together, they form the first protective shield.

Second Line of Defence: Non-Specific (Innate) Immunity

• If a pathogen sneaks past the first line, your innate immune system jumps into action. White blood cells like macrophages and neutrophils act as the “clean-up crew,” quickly identifying and attacking invaders. This response isn’t targeted—it attacks anything it sees as a threat. Signs of the second line at work include inflammation and fever, which help your body respond to infection.

Third Line of Defence: Specific (Adaptive) Immunity

• This is your body’s specialised response system, designed to target specific invaders. It relies on B lymphocytes to produce antibodies that recognise and bind to pathogens and T lymphocytes to destroy infected cells. The adaptive immune system also creates “memory cells,” which remember past invaders and respond faster if they return.

There are some great analogies for the immune system, like robbing a bank or protecting a castle. Choose an analogy to explain how the immune system works.