An example of your body's so-called first line of defense against infections would be

Although you may not think about it, anything you see on a person serves as a first line of defense.

The skin is almost always the first line of defense. Pathogens have a hard time going through skin, being on big shield. Also skin secrets enzymes and oils that destroy bacteria.

The orifices of the face particularly the nose and mouth use saliva and mucous to trap pathogen. Cilia further down pushes particles upward.

The cerumen ofbthe ear trap pathogens and particles really well.