**Core Concept**
Hyperacute rejection of a transplant is a rapid and severe immune response that occurs within minutes to hours after transplantation. It is primarily mediated by pre-existing antibodies against the donor's antigens, leading to activation of the complement system and subsequent tissue damage.

**Why the Correct Answer is Right**
Hyperacute rejection is caused by the presence of pre-formed antibodies against the donor's human leukocyte antigen (HLA) or ABO blood group antigens. These antibodies bind to the endothelial cells of the graft, triggering the activation of the complement system and the release of vasoactive mediators, which ultimately lead to thrombosis and ischemic damage to the graft. The endothelial cells express HLA class I and II molecules, which are the primary targets of the pre-formed antibodies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to hyperacute rejection, as it is not related to the immune response against the donor's antigens.
**Option B:** While activation of the complement system is a key component of hyperacute rejection, it is not the primary cause of this phenomenon.
**Option C:** Hyperacute rejection is not caused by an imbalance of the coagulation and fibrinolytic systems, but rather by the activation of the immune system against the donor's antigens.

**Clinical Pearl / High-Yield Fact**
Pre-existing antibodies against the donor's HLA or ABO blood group antigens can be detected pre-transplant using serological tests, and their presence can be a contraindication to transplantation.

**Correct Answer: B. Activation of the complement system.**