2.6: Physical Barriers to Disease

• The immune system is a complex network of organs, cells and proteins that defends the body from diseases and foreign substances • Detects when a virus, bacteria or foreigns substance is in the body • Causes scabs and blood to clot to prevent infection • The immune system keeps a record of every germ (microbe) it has ever defeated so it can recognise and destroy the microbe quickly if it enters the body again.

• Physical barriers that prevent entry to pathogens. For example: • Skin protects the internal organs • Fluids such as tears and saliva • Nose hairs to filter air • Stomach acid usually kills pathogens that have entered the digestive system • Vomiting and diarrhoea rapidly remove unwanted substances from the body

• Mechanical: Blinking, is a reflex response when the eye is irritated by foreign particles • Chemical: The cells in the eyes secrete enxymes called Lysozyme to chemically breakdown bacteria.

• Foreign particles are identified by the body as ‘non-self’ • Body’s own molecules and surface receptors are identified as ‘self’ • A particle identified as ‘non-self’ will be removed by antibody production