One of the basic themes of life is that many organisms consume or use other organisms to survive. For example, deer graze on grasses and wolves feed on deer. A parasite lives on or in another organism called the host. The host provides the parasite with the conditions and food necessary for survival. For example, hookworms can live in the sheltered environment of the human intestine, where they feed on blood. Humans are host to many different kinds of organisms, including microorganisms, such as bacteria, viruses, fungi, and protozoans; insects; and worms. It's often the case that parasites harm humans, causing disease and sometimes death. However, our bodies have ways to resist or destroy harmful microorganisms. The lymphatic system and immunity are the body's defense systems. This chapter considers the lymphatic system (p. 786), immunity (p. 796), innate immunity (p. 796), adaptive immunity (p. 801), immune interactions (p. 815), immunotherapy (p. 817), acquired immunity (p. 817), and the effects of aging on the lymphatic system and immunity (p. 820).