What is the primary cause of acute transplant rejection?

The primary cause of acute transplant rejection is T-cell activity causing tissue infiltration. Acute rejection typically occurs within the first few weeks to months after transplantation and is primarily mediated by the recipient's immune system.

When an organ is transplanted, the recipient's immune system recognizes the donor’s organ as foreign due to differences in human leukocyte antigens (HLA). T lymphocytes (T-cells) play a crucial role in this immune response. They become activated upon recognizing these foreign antigens, leading to the proliferation of T-cells that infiltrate the transplanted tissue. This infiltration results in a cellular immune response that can damage the graft, leading to rejection.

In contrast, the other options address different concepts related to transplant medicine. While HLA mismatch can lead to chronic rejection over a longer time frame, it is not the immediate cause of acute rejection. Renal vein thrombosis and urine leaks represent specific post-operative complications that could lead to graft dysfunction but are not the primary mechanisms of acute rejection itself.