Pathogens, agents that cause disease, infect a wide range of animals, including humansThe immune system recognizes foreign bodies and responds with the production of immune cells and proteinsAll animals have innate immunity, a defense active immediately upon infectionVertebrates also have adaptive immunityInnate immunity is present before any exposure to pathogens and is effective from the time of birthIt involves nonspecific responses to pathogensInnate immunity consists of external barriers plus internal cellular and chemical defensesAdaptive immunity, or acquired immunity, develops after exposure to agents such as microbes, toxins, or other foreign substancesIt involves a very specific response to pathogens