STEP 3: MATCH THE STEM CELLS WITH THE TRANSPLANT RECIPIENT

Our immune systems attack things they don't recognize, including cells and tissues. As with organ transplants, stem cell transplants can be rejected by the recipient's immune system. Therefore, the transplanted stem cells must match the recipient closely enough that they won't be recognized as intruders.

To determine whether the donor is a good immunological match with the recipient, a tissue typing test is performed using blood samples from both individuals. This test identifies certain proteins, called HLA antigens, which reside on the surfaces of specific immune cells. If the donor and the recipient have identical HLA antigens, they are a good match.

Rejection was a concern for the researchers developing the Parkinson's disease therapy. Previous research had told them, however, that immune responses are typically muted in the brain compared to other areas of the body. Therefore, they predicted that the fetal tissue would not trigger an intense immune response in the recipient.