The idea that a pathway of interaction existed between the brain and the immune system has been suggested for many years. However, only the evidence gathered over the past 25 years has provided important details about the mechanisms. The clues for bidirectional communication early on related to the effects of stress on immune function and that psychological factors could alter the onset and course of autoimmune disease (Ader 1996). A list of selected findings contributing to our understanding is shown in Table 1.1. The classical (Pavlovian) conditioning of host defense mechanisms and antigen-specific immune responses was first suggested and studied in the 1920s (Metal'nikov and Chorine 1926) and later in the 1970s (Ader and Cohen 1975). © 2007 Springer Science+Business Media, LLC. All rights reserved.