**Core Concept:** Allograft rejection refers to the immune response that occurs when a foreign tissue (allograft) is transplanted into a recipient's body. This can lead to damage or failure of the transplanted organ due to the host's immune system attacking the allograft as if it were a pathogen or tumor. There are two types of allograft rejection: primary and secondary. Primary allograft rejection occurs within the first few days to weeks following transplantation, whereas secondary allograft rejection occurs later, typically months to years after transplantation.

**Why the Correct Answer is Right:** Secondary allograft rejection is primarily mediated by **antibodies** produced by the recipient's immune system. These antibodies bind to antigens on the transplanted organ, activating a cascade of events that ultimately results in inflammation, tissue injury, and organ dysfunction. Antibodies are proteins produced by B cells, which are a type of white blood cell involved in the adaptive immune response.

**Why Each Wrong Option is Incorrect:**

A. **Antibodies are not involved in secondary allograft rejection:** As mentioned earlier, secondary allograft rejection is antibody-mediated. Therefore, this option is incorrect.

B. **T cells are the primary mediators of allograft rejection:** Although T cells play a role in allograft rejection, it is primarily the antibodies that initiate the cascade of events leading to tissue injury and organ dysfunction. Thus, this option is incorrect.

C. **Allograft rejection is always due to host immune system activation:** Although the host immune system is involved in allograft rejection, the primary mediators are antibodies, not T cells or other immune cells. This option is incorrect.

D. **Allograft rejection is solely due to donor immune system activation:** Like option C, this option is incorrect because both the host and donor immune systems contribute to allograft rejection, with antibodies being the primary mediators.

**Clinical Pearl:** The understanding of secondary allograft rejection mechanisms is crucial for transplant physicians to tailor appropriate immunosuppressive therapy and prevent organ dysfunction or failure. In practice, treating physicians usually administer a combination of immunosuppressive drugs, such as calcineurin inhibitors (e.g., cyclosporine and tacrolimus), antiproliferative agents (e.g., mycophenolate mofetil), and corticosteroids (e.g., prednisolone) to suppress the host's and donor's immune systems, thereby reducing the risk of allograft rejection.