**Core Concept**
An allograft is a type of graft that involves the transfer of tissue from one individual to another of the same species, but with genetic differences. This contrasts with an autograft, which involves the transfer of tissue from one part of the body to another in the same individual.

**Why the Correct Answer is Right**
Allografts are obtained from a donor who is genetically distinct from the recipient, often from a family member or an unrelated individual. This process is governed by the major histocompatibility complex (MHC) genes, which play a crucial role in the immune response. When an allograft is introduced into the recipient's body, the immune system recognizes the foreign antigens and mounts an immune response to reject the graft.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because an autograft involves the transfer of tissue from one part of the body to another in the same individual, whereas an allograft involves the transfer of tissue from one individual to another of the same species.
**Option B:** This option is incorrect because a xenograft involves the transfer of tissue from a different species to the same species, which is not the case with an allograft.
**Option C:** This option is incorrect because an isograft involves the transfer of tissue from one individual to another who is genetically identical, such as an identical twin.

**Clinical Pearl / High-Yield Fact**
It's essential to note that allografts are often used in transplantation surgery, such as kidney transplants, and require immunosuppressive therapy to prevent rejection. The success of allografts depends on matching the donor and recipient's MHC genes to minimize the risk of rejection.

**Correct Answer:** D