Defense Against Infectious Disease

Pathogens are microorganisms that cause disease, including bacteria, viruses, fungi, and protists. The body defends itself with a combination of physical and chemical barriers that block entry and internal immune responses that destroy pathogens that get inside.

Barriers to Infection

The skin is the body's largest organ and its first line of defense. Its tough outer layer of dead, keratinized cells forms a physical barrier that pathogens cannot easily cross, and secretions such as sebum and sweat are slightly acidic and inhibit microbial growth. Where the body has openings, other barriers operate. The airways are lined with mucus and cilia that trap and sweep away microbes, the stomach produces hydrochloric acid that kills many ingested pathogens, and tears and saliva contain the enzyme ly