It is well accepted that pregnancy establishes microchimerism. Lately, transmaternal passage of cells from elder siblings has been suggested as possible source of non-fetal microchimerism in nulliparous women. Recently, we identified both non-maternal microchimerism and minor H antigen specific cellular immune responses against these microchimeric cells in umbilical cord blood. The identification of the latter microchimeric cells from birth onwards explains the high incidence of microchimerism in healthy people and in people with different diseases. The circulating microchimeric cells of different origin can induce a wealth of antigen specific responses. Thus, nobody is born naïve: we are all microchimeras.