 The epidermis of the skin provides a physical barrier against pathogens, but it can easily be compromised by insect bites, animal bites, scratches or other minor skin trauma.Sweat, saliva, and tears are all chemical barriers that contain enzymes, such as lysozymes, that can kill bacteria and viruses.The mucus layer lines the nasopharynx and serves as a physical barrier that encases foreign pathogens and carries them back out of the body through snot and phlegm.