New York, Jan 19 (IANS): From the world's first facial 'replant' in 1994 on Sandeep Kaur, a nine-year-old Indian girl, to the now common full facial transplantation, the world has moved quite a lot.

But doctors are yet to understand many nuances of facial transplantation, especially the technical challenges -- how the recipient accepts or rejects the donated face.

According to a paper published in the journal "Modern Pathology", researchers at Brigham and Women's Hospital (BWH) in the US found that the immune cells, or T cells, of the donor - not necessarily of the recipient - play a key role in the rejection process of facial transplantation.

"The conventional belief about face transplant was that rejection is directly related to the recipient T cells attacking the donor T cells of the face, which are perceived as foreign to the recipient's immune system," said Christine Lian, a skin pathologist at BWH.

"We now need to rethink this process. Based on our findings, it is clear that the donor T cells, which are transferred as part of the new face, play a significant role in the rejection process as well," Lian said.

The researchers examined 131 face transplant biopsy specimens from a total of five patients who received face transplant between 2009 and 2013 at BWH.

The samples were examined by conventional microscopy for categorising the level of rejection and guiding immunosuppressant therapy, and additional antibody-based biomarkers were also applied.

Researchers found that during active rejection episodes, many immune cells in the face specimens that were involved in the rejection were of donor origin.

"The participation of these donor immune cells in face transplant rejection represents a paradigm shift in the understanding of the rejection process," said George F. Murphy, director of dermatopathology at BWH.

These new findings will help develop best diagnostic and therapeutic strategies that, for the first time, would consider include immune cells from the donor as well as the recipient.