This book explores the host pathogen interaction in plants, insects, and mammals to elucidate the origins of the human defense system against infection. Bringing together the latest research in the field of innate immunity, the authors review four significant research areas: plant immunity, invertebrate host defense immunity, pattern recognition receptors in mammalian host defense, and the links between innate and adaptive immunity in mammals. They find that the similarities among insect, plant, and mammalian immunological systems shed light on the complex mechanisms involved in adaptive immunity, thereby increasing our understanding of mammalian host defense.