If microbes get past your first line of defence, it is up to the second line of defence, your immune system, to keep you from getting sick.  You can’t see or feel your immune system because it is inside you.  It is made up of lots of different cells that are always patrolling your body and fighting any bad microbes they find.  Your body is exposed to many different types of microbes every day, so your immune system is always hard at work keeping you healthy! 

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