For an organ transplant, it is an advantage to have an identical twin.

Explanation

### Step-by-Step Solution:

1. **Understanding Organ Transplantation**: Organ transplantation involves taking an organ from one individual (the donor) and placing it into another individual (the recipient). The success of this procedure largely depends on the compatibility of the donor organ with the recipient's immune system.

2. **Role of the Immune System**: The immune system is designed to protect the body from foreign substances, including transplanted organs. When a foreign organ is introduced into the body, the immune system identifies it by the antigens present on the organ's surface.

3. **Antigen Matching**: If the antigens on the transplanted organ match those of the recipient, the immune system recognizes the organ as "self," leading to successful integration. However, if the antigens do not match, the immune system perceives the organ as foreign and initiates a rejection response.

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