Twenty-five years have passed since J. MILLER'S basic work on the
central role of the thymus for the generation of immunological reactivi-
ty. During this time, the abundance of fascinating literature on the
immunological and functional aspects of this organ has been in contrast
to the paucity of reports on its role in pathology. The causal or formal
pathogenesis even of many of the well-documented pathological fea- tures
is so far unexplained or at least uncertain. In spite of some concep-
tual progress related to the work of CASTLEMAN, LEVINE, and ROSAI, we
regrettably have to say that in pathology the thymus remains almost as
inconspicuous as 25 years ago. Only the new techniques developed in
recent years have made it possible to look as closely into the
complicated structural organization of the thymus as it appears to be
necessary to observe and document abnormalities. Major steps have been
taken with the advent of mono- clonal antibody immune histochemistry and
with detailed ultrastructur- al studies in embryology. At this point,
pathologists and researchers with a special interest in the thymus were
asked to give overviews of their respective fields of interest in light
of recent findings in immunology and basic insights into the
structural-functional interrelationship of the human thymus. The results
of this initiative have been brought together in this volume.