The study of immunology encompasses a vast and ever-growing body of information that in some way or other incorporates most areas of medical biological research. As the body of information in the medical sciences continues to increase its rate of expansion, one of the greatest challenges to investigators will be to integrate this information in a manner that is intellectually fruitful and productive. Considering the intended scope of this text, we could not pretend to have gone too far toward achieving such an integration—and considering the pace of change, in its very best form a measured approximation of such lofty goals might be the most we could hope for. Nevertheless, in these pages we have sought to produce a collection of information that is at once concise and up-to-date regarding areas where important developments are impacting on the way we understand the vertebrate immune system. In addition, although the information is geared toward advanced study, we have discussed some basic elements and concepts that we hope make the text a useful resource for both the immunologist and the nonspecialist. The intention is to provide the researcher, clinician, or advanced undergraduate student with a brief ov- view of specific components of the immune system, and to provide a place from which to begin further detailed study if necessary. To this end, we made every effort to supply extensive referencing—although limitations in space prevented exhaustive or complete referencing in some cases.