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

Analysis on five common classes of human natural chimeras (cytomictical, wh ole body, fetal-maternal, germ cell, and tumor chimeras) reveals that (1) t hey initiate only during pregnancy, (2) the most common class are chimeras which contain maternal cells, and (3) the primary mechanisms that are invol ved in their formation and establishment are still elusive. These classes o f natural chimerism, are involved only with maladaptive phenomena such as m alignancy and autoimmune diseases and without ally documented benefit. A re cent review has challenged the accepted dogma that the evolution of immunit y is pathogen-directed and asserted that preserving individuality from litt ering the soma and the germline by conspecific alien cells might have been the original function of the innate immunity. Following this tenet, I propo se here chat human natural chimerism is a byproduct of che new role evolved from primitive components of immunity to "educate" the developing embryo w ith the armamentarium of effector mechanisms, dedicated co purge the indivi dual from pervasive somatic and germline variants, and is not a vestige of evolution. Human Immunology 62, 651-657 (2001). (C) American Society for Hi stocompatibility and Immunogenetics, 2001. Published by Elsevier Science In c.