**Question:** Hyperacute graft rejection is caused by -

A. Antibodies
B. Immune cells
C. Cytotoxic T cells
D. Complement activation

**Core Concept:**
Hyperacute graft rejection is a severe form of organ transplant rejection that occurs within minutes to hours of organ transplantation. This type of rejection is primarily mediated by antibodies and complement activation.

**Why the Correct Answer is Right:**
Hyperacute graft rejection is a result of pre-formed antibodies against the donor's antigens on the surface of the transplanted organ. These antibodies, combined with the complement system, lead to the destruction of the transplanted organ due to immune-mediated direct cellular damage. Complement activation plays a crucial role in amplifying the immune response and enhancing the cytotoxicity of antibodies and immune cells.

**Why Each Wrong Option is Incorrect:**

A. Immune cells (e.g., T cells, B cells) alone cannot cause hyperacute graft rejection since they require specific antibodies against the donor's antigens for activation.

B. Immune cells are involved in acute and chronic graft rejection but not hyperacute rejection due to the presence of preformed antibodies in hyperacute rejection.

C. Cytotoxic T cells are involved in acute cellular rejection, not hyperacute graft rejection, which relies on antibodies and complement activation.

D. Complement activation contributes to hyperacute graft rejection, making option D partially correct. However, the primary cause of hyperacute rejection is the presence of preformed antibodies against the donor's antigens on the transplanted organ.

**Clinical Pearl:**
Understanding the mechanisms of hyperacute graft rejection is crucial for transplant surgeons, as it highlights the importance of proper matching of human leukocyte antigens (HLAs) between the donor and recipient to minimize the risk of hyperacute graft rejection. Pre-emptive measures, such as using immunosuppressive drugs or cadaveric organ donation, can help prevent this severe form of rejection.

**Correct Answer:** D. Complement activation