BIOLOGY Volume 2 - A Guide to General Biology - 2004

14. TRANSPORT IN ANIMALS

14.9. The Immune System

14.9.9. Tissue and Organ Transplantation

Transplantation refers to the replacement of damaged Tissues and Organs with healthy ones. In recent years, transplantation has become increasingly widespread in surgical practice. However, transplanted foreign tissue is rejected by the recipient's body because the graft acts as an antigen that triggers an Immune Response.

Types of Grafts

The following terms are used to describe various types of grafts.

Autograft — tissue transplanted from one site on the body to another in the same individual. Rejection does not occur. This is commonly used in Skin grafting.

Isograft — tissue transplanted from a genetically identical Organism, such as an identical twin. Rejection also does not occur.

Allograft (homograft) — tissue transplanted from one individual to another of the same species but with a different genotype.

Xenograft — tissue transplanted from an individual of one species to an individual of another species, such as from a pig to a human.

Rejection

In this section, we will focus exclusively on allografts.

The simplest example of an allograft is a Blood transfusion. In fact, blood is a fluid tissue, so any blood transfusion can be regarded as a tissue allograft. In this case, rejection manifests as the agglutination of donor red Blood Cells. However, finding a suitable blood donor is relatively straightforward because only two Antigens (A and B) are critical. Problems arise with other tissues, whose cells carry numerous antigens encoded by genes that make up the so-called Major Histocompatibility Complex (MHC). Graft rejection related to this complex is discussed in detail in Section 25.7.13.

We will examine the typical mechanism of the rejection process using the recipient's response to a skin allograft as an example. This involves the following sequence of events.

1. Over the first 2–3 days, Blood Vessels develop within the allograft (a process known as vascularization), and the graft appears healthy.

2. Over the subsequent six days, vascularization of the graft decreases, and A large number of killer T cells and macrophages accumulate around it.

3. After another two days, the graft cells begin to die, and eventually the graft sloughs off.

Prevention of Allograft Rejection

Several Methods are currently used to prevent allograft rejection.

1. Selection of the most compatible tissue is an obvious and necessary precaution that must be taken in any transplantation. This involves analyzing the major histocompatibility complex (Section 25.7.13). The most suitable donor in such cases should be sought among the recipient's close relatives.

2. Irradiation of the recipient's Bone Marrow and lymphoid tissues with X-rays. This suppresses The production of white blood cells, including T lymphocytes, thereby slowing down tissue rejection. However, this disrupts the overall protective function of the blood, increasing the risk of infection. Furthermore, undesirable side effects may occur.

3. Immunosuppression — the non-specific suppression of The Immune System using certain chemical agents. While this slows down graft rejection, it introduces another serious problem: the recipient becomes susceptible to any infection. It has also been shown that immunosuppression increases the risk of developing Cancer.

4. One way to reduce the side effects of irradiation and immunosuppression is the targeted destruction of only those cells that directly respond to the antigens of the specific graft, namely killer T cells. In this case, the rest of the immune system continues to function normally. The most promising approach involves administering Monoclonal Antibodies to the recipient (via blood or bone marrow) that specifically recognize and destroy only the killer T cells responsible for graft rejection. Highly effective monoclonal antibodies that prevent Kidney transplant rejection have already been developed (see Section 12.11.2).