The first line of defense (or outside defence system) includes physical and chemical barriers that are always ready and prepared to defend the body from infection. These include your skin, tears, mucus, cilia,stomach acid, urine flow, ‘friendly’ bacteria and white blood cells called neutrophils.

Pathogenic (disease-causing) microorganisms must make it past this first line of defence. If this defense is broken, the second line of defense within your body is activated.

Skin The skin is the largest organ of your body. It acts as a barrier between invaders (pathogens) and your body. Skin forms a waterproof mechanical barrier. Microorganisms that live all over your skin can’t get through your skin unless it’s broken.

Tears, mucus and saliva Your nose, mouth and eyes are obvious entry points for pathogens. However, tears, mucus and saliva contain an enzyme that breaks down the cell wall of many bacteria. Those that are not killed immediately are trapped in mucus and swallowed. Special cells line and protect the nose, throat and other passages within your body. The inner lining of your gut and lungs also produces mucus to trap invading pathogens.

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Immune System: 2. The foot soldiers
What is Snot?