In the skin, there is a thick layer of dead cells in the skin that act as a barrier- like the gate is to the castle! With the gate intact, the army can't get in!
As this layer of cells shed, disease causing cells, called microbes, are removed by Mucous membranes – Mucous membranes produce mucus to trap these cells so they cannot spread to the rest of the body.
Another form of a physical barrier is hair- nose hair! It may be gross, but it's pretty helpful when fighting infections! These nose hairs filter out microbes, and they also filter dust and harmful infections from the air to help prevent them from invading the body.
The second line of defense, after the physical barrier, is called nonspecific resistance. This destroys the invaders in a general way, less targeted.
In nonspecific defense, there is something called phagocytes, cells that destroy diseases that pass into our body tissues.
Also in nonspecific defense, inflammation is a response in the body tissue that happens when tissues are damaged or in response to other harmful reactions in your body. This is like if the army got into the castle, we have guards to protect us!