A cornea transplant is never rejected in humans because

The correct option is "it has no blood supply".

A cornea transplant (corneal grafting) is one of the most successful transplant procedures in humans. The primary reason for its extremely high success rate and lack of rejection is immunological privilege, which is explained step-by-step below:

1. Avascular Nature of the Cornea:
The cornea is the transparent outer layer of the eye. To remain clear and allow light to pass through unobstructed, the normal cornea is completely avascular, meaning it contains no blood vessels.

2. Absence of Immunological Surveillance:
In typical organ transplants (like kidney or heart), the recipient's immune system detects the foreign antigens on the donor tissue through immune cells carried in the blood. Since the cornea has no blood supply, the recipient's lymphocytes (white blood cells) and antibodies cannot reach the transplanted tissue to identify it as foreign.

3. Lack of Lymphatic Drainage:
Similarly, the cornea lacks lymphatic vessels. This prevents donor antigens from traveling to the regional lymph nodes, where an immune response would normally be initiated and sensitized.

Conclusion:
Because the cornea lacks a blood supply, it is isolated from the body's active immune surveillance. This prevents the immune system from mounting a rejection response, allowing the transplant to succeed without the tissue-matching requirements typical of other organs.