Chapter 43 The Immune System Overview: Reconnaissance, Recognition, and Response Barriers help an animal to defend itself from the many dangerous pathogens it may encounter. The immune system recognizes foreign bodies = ?not self? and responds with the production of immune cells and proteins. Two major kinds of defense have evolved: innate immunity and acquired immunity. How do immune cells of animals recognize foreign cells? 1.5 ?m Innate immunity is present before any exposure to pathogens and is effective from the time of birth. It involves nonspecific responses to pathogens. Innate immunity consists of external barriers plus internal cellular and chemical defenses.