The following chapter reviews principles of immunology to provide an understanding of how components of the nervous system are recognized by the immune system, how an autoimmune response is mounted, how immune cells and mediators enter the nervous tissue, and how tolerance against neural antigens is induced, maintained, and broken. Most of the general principles have been discovered since the 1980s but only with the new technology of targeted deletions and mutations (permanent and conditional knockout, knockin) can these principles be systematically explored at the molecular level. A brief discussion of multiple sclerosis, the Guillain–Barré syndrome, and myasthenia gravis will follow, while clinical aspects and disease-specific pathomechanisms of immune-mediated neurological disorders are presented in greater detail in individual chapters. Therapeutic consequences based on immunological principles are discussed in Chapter 93.
Categories of the immune response
The immune system is a multifaceted system of cells and molecules with specialized tasks in defending the organism from external agents, infectious or toxic. Moreover, the immune system plays a pivotal role in maintaining antigenic homeostasis in the body. Two types of responses to invading organisms can take place: an acute response launched within hours, and a delayed response occurring within days. The immediately responding system is termed innate immune system, and it evolves stereotypically and at the same magnitude regardless how often the infectious agent is encountered. In contrast, a more delayed response is delivered by the adaptive or acquired immune system and provides a more specific immunologic reaction which improves in efficiency on repeated exposure to a given infective agent, capitalizing on the formation of immunological memory. The immune system has traditionally been divided into innate and adaptive systems, each containing different cellular and molecular components. The main distinction between these two systems lies in the mechanisms and receptors used for immune recognition. These two systems are not separated, but are functionally connected allowing for intensive interactions (Carroll & Prodeus, 1998; Ochsenbein & Zinkernagel, 2000).
The innate immune system
During evolution, the innate immune system appeared before the adaptive immune system, and some form of innate immunity probably exists in all multicellular organisms. Characteristically, innate immune responses consist of all the immune defence mechanisms that do not require immunologic memory. Genetically, the molecular mediators and their receptors are highly conserved between species as far apart as Caenorhabditis elegans, Drosophila, and mammals.