Fighting Disease The controlled environment within an animal's body is a comfortable place for hostile invaders as well as for its own cells. Most environments contain disease-causing microorganisms, or pathogens, that may take advantage of steady supplies of oxygen and nutrients intended for body tissues. If pathogens enter the body and grow, they may disrupt homeostasis in ways that cause disease.

Most animals have an immune system that can distinguish between “self” and “other.” Once the immune system discovers “others” in the body, it attacks the invaders and works to restore homeostasis. Your body experiences this process regularly, any time you catch a cold or fight off other kinds of infections. During the process, you may develop a fever and feel other effects of the battle going on within your body.

Chemical Controls Vertebrates, such as the migrating wildebeest, along with arthropods and many other invertebrates, regulate many body processes using a system of chemical controls. Endocrine glands are part of that system. Endocrine glands regulate body activities by releasing hormones into the blood. Hormones are carried by blood or body fluids to organs. Some hormones, as you have learned, control growth, development, and metamorphosis in insects.

Mammals, like other vertebrates, have endocrine glands that are part of an endocrine system. Some hormones control the way the body stores energy or mobilizes it—as in the case of the wildebeests. Other hormones regulate the amount of water in the body and the amount of calcium in bones.

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