Final answer:
Transplant rejections primarily occur due to differences in HLA antigens, leading to an immune response against the donor tissue.
Step-by-step explanation:
The primary reason for transplant rejections is due to differences in HLA antigens between the donor and recipient.
When a donor organ expresses MHC molecules that are different from those of the recipient, the recipient's immune system may recognize these molecules as foreign and mount a cytotoxic T cell response, resulting in transplant rejection.
This process is a classical primary T cell immune response and can lead to organ failure if not managed with immunosuppressive therapy and careful tissue typing to ensure the best possible match.