When homeostasis is disturbed or threatened, a repertoire of molecular, cellular and behavioral responses comes into play. These responses attempt to counteract the disturbing forces in order to reestablish a steady state. They can be specific to the foreign invader or a particular stress, or they can be generalized and nonspecific when the threat to homeostasis exceeds a certain threshold. The adaptive responses may themselves turn into stressors capable of producing disease. We are just beginning to understand the many ways in which the brain and the immune system are interdependent, how they help to regulate and counterregulate each other and how they themselves can malfunction and produce disease.
The stress response promotes physiological and behavioral changes that enhance survival in threatening or taxing situations. For instance, when we are facing a potentially life-threatening situation, the brain’s stress response goes into action to enhance our focused attention, our fear and our fight-or-flight response, while simultaneously inhibiting behaviors, such as feeding, sex and sleep, that might lessen the chance of immediate survival. The stress response, however, must be regulated to be neither excessive nor suboptimal; otherwise, disorders of arousal, thought and feeling emerge.
The immune system’s job is to bar foreign pathogens from the body and to recognize and destroy those that penetrate its shield. The immune system also must neutralize potentially dangerous toxins, facilitate repair of damaged or worn tissues, and dispose of abnormal cells. Its responses are so powerful that they require constant regulation to ensure that they are neither excessive nor indiscriminate and yet remain effective. When the immune system escapes regulation, autoimmune and inflammatory diseases or immune deficiency syndromes result.
The immune and central nervous systems appear, at first glance, to be organized in very different ways. The brain is usually regarded as a centralized command center, sending and receiving electrical signals along fixed pathways, much like a telephone network. In contrast, the immune system is decentralized, and its organs (spleen, lymph nodes, thymus and bone marrow) are located throughout the body. The classical view is that the immune system communicates by releasing immune cells into the bloodstream that float, like boats, to new locations to deliver their messages or to perform other functions. The central nervous and immune systems, however, are in fact more similar than different in their modes of receiving, recognizing and integrating signals from the external environment and in their structural design for accomplishing these tasks. Both the central
nervous system and the immune system possess “sensory” elements, which receive information from the nvironment and other parts of the body, and “motor” elements, which carry out an appropriate response.
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