First line of defence

The first line of defence of any city is it’s walls and sentries and the human body is no different.

Just existing in an environment leaves every body open to infiltration by unwanted foreigners. Dust, dirt, pollen, spores, bites, stings, virus’s, bacteria’s and the odd solid object, the body needs to deal with and deny access to the delicate internal system before they get a chance to become nasties and do damage.