Researchers said the cell plays a key role in preventing organ rejection. The cell could also aid the body's transplant tolerance.
Scientists hope the discovery will help them identify patients who no longer need to take immunosuppressant drugs, which can have unpleasant side affects particularly over long periods.
They will now be looking to see if the same cells are present in liver and kidney transplant recipients who have been successfully weaned off their immunosuppresants and in patients who are exceptionally tolerant of their new organs.
Dr Angus Thomson, professor of surgery and molecular genetics and biochemistry at the University of Pittsburgh's School of Medicine and the Thomas E Starzl Transplantation Institute, said: "This will provide a 'roadmap' for clinicians to help identify those for whom the immunosuppression can be safely withdrawn.
"We'd like to identify the cellular and molecular clues so that assays, or very simple laboratory tests, can be developed that would be predictive of tolerance."
The special cell, a subtype of a dendritic cell - a rare type of white blood cell present in all tissue - was originally thought just to prompt the rejection process.
'Significant' findings
Dendritic cells are known for their ability to identify and target foreign substances for destruction of other cells within the immune system.
But some dubbed "good dendritic cells" have the opposite effect.
Dr Peta O'Connell told Transplant 2001, the joint meeting of the American Society of Transplantation (AST) and the American Society of Transplant Surgeons, that the findings were "significant".
She said experiments giving mice a pre-transplant of lymphoid dendritic cell sub-types, derived from tissues like the spleen, had been shown to prolong a heart transplant in a mouse model, even without the drugs being given.
In contrast the myeloid dendritic cells, those relating to the bone marrow, actually accelerated the rejection response.
Scientists now hope to harness the "good dendritic" cells to improve acceptance of transplant organs.
Dr O'Connell said: "The lymphoid-derived dendritic cells somehow disarm immune system T cells from doing their part to attack the donor organ possibly by causing either their death or limiting their proliferation."
Professor David Oliveira, professor of renal medicine at St.George's medical school, said the research opened up exciting possibilities.
"It certainly raises the possibility of using the cells to induce tolerance, but I suspect it is a long way off."