Tyler "Chimera" Hamilton

A chimera is defined as an organism whose cells originate from two or more
zygotes. The definition of blood chimerism requires the presence of cells
derived from two (or more) genetically different individuals populating the
lympho-haematopoietic compartments. Blood chimerism may occur artificially
or spontaneously. Artificial chimerism occurs transiently after blood
transfusions, while allogeneic bone marrow or stem cell transplantation
results in stable blood chimerism. The spontaneous type of blood chimerism
has been observed in twins after transfusion of haematopoietic stem cells
between fetuses during the first trimester. Placental injection studies
revealed frequent anastomoses of blood vessels between monochorionic and
monozygotic twins. However, in human dizygotic twins, cell exchange with
proliferation in the genetically different fetus is rare, and only a few
cases have been described. Recent findings point to a more frequent
incidence of blood chimerism in multiple births.