Chapter 43 The Immune System Overview: Reconnaissance, Recognition and Response Pathogens: infectious agents that cause disease Constantly attacking animals Infect a wide variety of animals (including humans) Types of pathogens: Viruses Bacteria Protists Fungi Immune system: The animal?s way of fighting off the Pathogens Made up of many different things (Macrophages, proteins, etc.) Layers of Defense: Outer layer: (barrier) Such as skin or shell Significant obstacle to invasion by microbes(pathogens) Not fully sealed (b/c need to breathe, etc) Additional barrier defenses: such as chemical secretions that trap/kill microbes ? guard the body?s entrances and exits.