Types of grafts and their fate in the body
The success of any organ or tissue transplantation surgery directly depends on the origin of the biomaterial used. The recipient's immune system reaction will differ radically depending on the degree of genetic relatedness.
Biology and medicine distinguish the following main types of grafts:
- Xenografts are materials obtained from organisms of another biological species. They are perceived by the immune system as completely foreign and undergo inevitable and rapid rejection.
- Allografts are organs and tissues transplanted from individuals of the same species who do not have a direct genetic relation to the recipient. This is the most common option in clinical practice, which also triggers a strong defense reaction and is rejected without special medical intervention.
- Isografts are tissues obtained from genetically related individuals. Due to the high degree of genetic similarity, they are usually not rejected and successfully engraft.
- Autografts are the body's own tissues moved from one part of the body to another.
Mechanisms of foreign tissue rejection
Transplanted organ rejection is a complex yet classic immunological process. The recipient's immune system is activated in the exact same way as it reacts to dangerous viruses, bacteria, or toxins entering the body.
The main cause of rejection is a mismatch of histocompatibility antigens. These are specific molecules present on the surface of donor cells. Their presence and structure are especially easy to determine when examining the cell membranes of lymphocytes. As soon as the recipient's immune cells recognize these antigens as foreign, a cascade of reactions is triggered aimed at destroying the graft cells.
Methods for suppressing the immune response
To preserve the transplanted organ and prevent its failure, modern medicine uses an entire arsenal of methods aimed at combating rejection. First and foremost, thorough tissue typing is performed—matching the optimal donor and recipient based on 7 key histocompatibility genes. These genes are localized as a single cluster on chromosome 6. Their main feature is extremely high polymorphism and multiple allelism (up to 47 different alleles are known today). It is precisely because of such genetic diversity that the search for a donor is extremely difficult, requiring the creation of large-scale international recipient databases.
If full compatibility cannot be achieved, methods of artificial immune suppression are used:
- Immunosuppression — systematic use of pharmacological immunosuppressants.
- Radiation exposure — targeted or total X-ray irradiation to reduce immune cell activity.
- Surgical methods — radical removal of organs responsible for immunity (spleen or thymus).
- Hematological methods — full blood transfusion (fairly rarely used in modern practice).
Alternative approaches: alloplants and prosthetics
One of the promising areas is the stimulation of reparative regeneration using alloplants. Alloplants are unique biological materials characterized by extremely weak immunogenicity. The technology of their creation includes special processing of donor tissues (such as the eye sclera or tendons) with glycosaminoglycans.
Such processing not only significantly reduces the risk of rejection but also stimulates the process of replacing the transplanted graft with the patient's own newly formed healthy tissue. In medical practice, the following are successfully used:
- Hyaluronic acid-enriched allplant — used to reconstruct the eyelid framework.
- Heparan sulfate allplant — used for eye conjunctiva plastic surgery, skin transplantation, and surgical reconstruction of the auricle.
In situations where the use of biological tissues is inappropriate or impossible, doctors resort to organ and tissue prosthetics. Biological structures are replaced with reliable artificial counterparts. Among them, pacemakers, artificial heart valves, high-tech joint prostheses, as well as large blood vessel prostheses made of bioinert materials (teflon and silicone) have become the most widespread.
Briefly: answers to topical questions
What classes of major histocompatibility complex (MHC/HLA) antigens are distinguished in humans?
In humans, the sources explicitly indicate the following HLA/MHC:
- MHC class I antigens — HLA-A, HLA-B, HLA-C.
- MHC class II antigens — HLA-DR, HLA-DP, HLA-DQ.
Separately in the functional classification of MHC genes, MHC class III is distinguished: these genes encode soluble immune mediators that do not directly participate in antigen presentation.
Non-classical MHC-I molecules (subclass Ib) are also described: HLA-E, HLA-G, to a lesser extent HLA-F.
Which groups of pharmacological drugs are used as immunosuppressants in transplantation?
In the context of pharmacological immunosuppressive therapy for transplantation/graft rejection, sources explicitly mention:
- selective immunosuppressants — used as induction therapy or to manage rejection crises;
- mycophenolate-group immunosuppressants;
- specific drugs: cyclosporine and tacrolimus.
Broadly speaking, the use of pharmacological immunosuppressants to suppress graft rejection is also indicated.
What types of graft rejection are distinguished depending on the time of their development?
Depending on the timing of the reaction's development, three types of graft rejection are distinguished.
- Hyperacute rejection develops in the early stages, on days 1–5 after surgery.
- Acute rejection occurs within a period of 5 to 30 days.
- Chronic rejection forms at late stages, 6 months or more later.
Class and exam questions
What is the root cause of transplanted organ rejection?
The main cause of rejection is a mismatch of specific histocompatibility antigens located on the surface of cells (in particular, on lymphocytes) of the donor and recipient.
Why is it so difficult to find a suitable donor for transplantation?
The difficulty lies in the high polymorphism and multiple allelism of histocompatibility genes (up to 47 alleles have been identified). That is why international databases are created to find compatible pairs.
What is the specific feature of using alloplants?
Alloplants undergo special processing with glycosaminoglycans, due to which they possess very weak immunogenicity. Instead of rejection, they stimulate the replacement of the graft with the body's own tissue.
Which artificial organ analogues are used in prosthetics?
Instead of biological grafts, artificial heart valves, pacemakers, joint prostheses, and large vessel prostheses made of teflon or silicone are often used.
What else to study on this topic
- Genetic control of tissue compatibility: gene cluster on chromosome 6.
- Mechanism of action of pharmacological immunosuppressants.
- Surgical immunosuppression: the role of the thymus and spleen in cellular immunity.
- The role of glycosaminoglycans (hyaluronic acid and heparan sulfate) in reparative regeneration.
- Biocompatibility of artificial materials: teflon and silicone in prosthetics.
Master the entire topic in the "Biology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Transplant immunity" and 3,793 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 129 video lessons on the subject
Other topics in the "Transplantation" section
Free video lessons: BiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.