Some analogies might help differentiate how chemical signaling in the immune, nervous, and endocrine informational systems works. The immune system (the topic of Chapter 42) operates like an army of private security guards. Immune system cells make their rounds of the body, and if they detect a security breach, they sound their alarms—cytokines—that activate the body’s defenses. The nervous system (see Chapters 44, 46) operates like a landline telephone system, with a central integration and command center that sends signals along specific wires to specific receivers. The endocrine system is more like a radio, television, or cell-phone system, broadcasting signals that can be picked up by anyone who has an appropriate receiver that is turned on and tuned in. Some endocrine signals are weak like those of a walkie-talkie and are picked up only locally, but others are strong like those of a major broadcast station and reach distant parts of the body. In all cases, the signals are received only by cells that have appropriate receptors, and the responses are determined by the receiving cell’s internal signal transduction machinery.