Human natural chimerism: An acquired character or a vestige of evolution?

Analysis on five common classes of human natural chimeras (cytomictical, whole body, fetal-maternal, germ cell, and tumor chimeras) reveals that (1) they initiate only during pregnancy, (2) the most common class are chimeras which contain maternal cells, and (3) the primary mechanisms that are involved in their formation and establishment are still elusive. These classes of natural chimerism, are involved only with maladaptive phenomena such as malignancy and autoimmune diseases and without ally documented benefit. A recent review has challenged the accepted dogma that the evolution of immunity is pathogen-directed and asserted that preserving individuality from littering the soma and the germline by conspecific alien cells might have been the original function of the innate immunity. Following this tenet, I propose here chat human natural chimerism is a byproduct of che new role evolved from primitive components of immunity to educate the developing embryo with the armamentarium of effector mechanisms, dedicated co purge the individual from pervasive somatic and germline variants, and is not a vestige of evolution. Human Immunology 62, 651-657 (2001). (C) American Society for Histocompatibility and Immunogenetics, 2001. Published by Elsevier Science Inc.