Cornea transplant in human is almost never rejected. This is because:

Options

A. It is composed of enucleated cells
B. It is a non-living layer
C. Its cells are least penetrable by bacteria
D. It has no blood supply

Answer

**Step-by-Step Solution:**

1. **Understanding Transplantation**: Transplantation refers to the process of transferring an organ or tissue from one body (donor) to another (recipient). This can often lead to graft rejection if the recipient's immune system recognizes the donor tissue as foreign.

2. **Graft Rejection Mechanism**: The immune system plays a crucial role in graft rejection. It identifies the donor tissue based on unique proteins called Human Leukocyte Antigens (HLA). If the recipient's immune system perceives these proteins as foreign, it activates T killer cells to attack and destroy the transplanted tissue.

3. **Cornea Characteristics**: The cornea is a transparent layer of tissue that covers the front of the eye. It is unique among tissues because it has no blood supply. This characteristic is significant in the context of transplantation.

4. **Why Cornea Transplants Are Not Rejected**: The absence of blood supply in the cornea means that lymphocytes (the white blood cells responsible for immune responses) are not present in this tissue. Since T and B cells, which are crucial for triggering immune responses, are found in the blood, the lack of blood supply in the cornea reduces the likelihood of an immune response against it.

5. **Conclusion**: Therefore, the reason cornea transplants are almost never rejected is primarily due to the absence of blood supply, which prevents the presence of immune cells that could trigger rejection.

**Final Answer**: The cornea transplant in humans is almost never rejected because it has no blood supply.

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