[Transplantation immunology].

This Special Focus on Transplantation Immunology issue of Chimerism features papers focusing on the role of chimerism in transplantation immunology. The field of immunologic tolerance, which owes its origins to an experiment of nature resulting in hematopoetic mixed chimerism, is the subject of our featured Forum article—a debate between Jeff Mold and Colin Anderson on the implications for tolerance of the layered development of the human immune system (p. 62). Jeff argues that different species (e.g., mice vs. humans) have different solutions to achieve tolerance, with a preponderance of Treg in fetal life being critical for primates, while Colin considers the primary rules for tolerance are instead conserved and argues for the primacy of central tolerance. The impact of fetal chimerism resulting from pregnancy on the outcome of organ transplantation in women of childbearing age is the subject of a Cutting Edge View Transplantation immunology