Question

.Cornea transplant in humans is almost never rejected is because (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.

Short answer

Cornea transplants are rarely rejected because the cornea has no blood supply, limiting the immune system's ability to mount a rejection response.

Step-by-step solution

Understand the basis of tissue rejection

Tissue rejection in transplants typically occurs when the recipient's immune system identifies the transplanted tissue as foreign and mounts an immune response against it. To understand why cornea transplants are rarely rejected, consider factors that reduce immune detection or response.

Examine the given options

Review each of the provided options to determine which factor is most relevant to the low rejection rate of corneal transplants. Analyze the options in relation to the immune system and the specifics of corneal tissue.

Identify the correct option

Option (d) 'it has no blood supply' is the correct answer because the absence of blood vessels in the cornea means that immune cells and antibodies have limited access to the transplanted tissue, hence reducing the likelihood of a rejection response.

Key concepts

Tissue Rejection

Tissue rejection is a significant concern when it comes to transplants, as the body's immune system is always on the alert for anything it considers to be foreign. A transplant is typically seen by the body as an intruder because it comes with cells that have different proteins called antigens on their surfaces. These antigens are like flags that tell the immune system, 'Hey, I'm not from around here!' In an attempt to protect the body, immune cells swing into action, targeting the transplanted tissue for destruction.

This immune battle can lead to what's called a rejection, which is when the body doesn't accept the new tissue and tries to get rid of it. To increase the chances of a successful transplant, doctors match the tissue types as closely as possible and use immunosuppressive drugs to dampen the immune response. Yet, in the case of corneal transplants, rejection is rare because the cornea doesn't wave those 'foreign' flags as prominently due to its lack of blood vessels.

Immune Response

The immune response is the body's defense mechanism against invaders, such as viruses, bacteria, and transplanted tissues. When the immune system detects something foreign, it triggers a complex biological response. This includes sending out specialized cells and proteins to attack and disable the invader.

When it comes to transplants, the immune response can turn into a double-edged sword. It's designed to protect us, but it can also threaten the survival of a transplanted organ or tissue by mistaking it as a threat. The intensity and type of immune response are dictated by factors such as the presence of blood vessels, which are channels for immune cells and antibodies to reach and attack transplanted tissue. With corneal transplants, the limited blood supply plays a role in minimizing these immune responses, leading to a higher success rate compared to other transplants.

Corneal Tissue

Corneal tissue is unique in several ways. It's the clear, dome-shaped surface that covers the front of the eye and has a crucial role in focusing our vision. Unlike most tissues in the body, the cornea is avascular, meaning it doesn't contain blood vessels. This is important not only for transparency but also when it comes to transplants.

Because the cornea doesn't have its own blood supply, it doesn't have direct access to the immune system's cells and molecules circulating in the bloodstream. This avascularity is a key reason why corneal transplants are less likely to be rejected. Without blood vessels, the immune system 'patrols' are less likely to find the transplanted tissue and start an immune attack, helping the transplanted cornea to survive and function in its new environment.

Blood Supply in Transplants

Blood supply plays a vital role in the survival of transplanted tissues and organs. Most tissues in the body need a direct blood supply to deliver oxygen and nutrients and to remove waste products. However, blood vessels also serve as highways for the cells and proteins of the immune system. When a transplant contains blood vessels, it's like giving an all-access pass to immune cells, which can lead to an aggressive immune response and potential rejection.

In transplants where the blood supply is limited or non-existent, like in corneal tissue, the chances of the immune cells encountering the new tissue are significantly reduced. This lack of blood supply to the cornea is what makes it such a tolerant transplant tissue, avoiding the full-blown immune assaults that other transplants might provoke. Thus, the success rates for corneal transplants are generally higher, thanks to this unique biological feature.