This chapter describes the brain responses to antigenic challenges. The functions of the immune and nervous systems were considered separately until the middle of the twentieth century, at which time the accumulation of experimental and clinical evidence of their interactions stimulated the emergence of a novel scientific discipline—immunophysiology. Studies of the electrical activity of brain structures upon antigen administration have made it possible to determine the pattern of their involvement in different stages of immune responses; the sequential changes in the activities of certain structures have provided evidence of a preferential involvement of some of them in brain responses to antigens at defined stages of immunogenesis. Thus, studies of brain function over the course of immune responses provide the grounds for a transformation of the understanding of influences exerted by the brain on immunogenesis into the notion of the neural regulation (modulation) of immune functions. Current developments in immunophysiology are marked by inquiries into the molecular–biological mechanisms that underlie the interrelationships between the nervous and immune systems and into the role of the immune system in brain function.