Transplantation is a process of picking some cells, tissues or organs from one site and placing them at another site in same or different individuals. It is generally carried out to replace a diseased organ by a healthy one. The transplantation may sometimes lead to various complications in the host, which are mediated through host body immune responses.
What are transplantation antigens?
Transplantation antigens are those antigens that are present on the cell surface and are responsible for graft rejection or acceptance. These antigens induce the immune response in the host that may cause the rejection of the transplanted tissue.
The Major Histocompatibility Complex (MHC) antigens are the major barriers to transplantation of nucleated cells. MHC antigens are surface molecules that direct T-cells to carry rapid graft rejection. These antigens are termed as Human Leukocyte Antigen or HLA Complex in Humans and the gene coding for the molecules are present on Human Chromosome 6. These genes encode a very large number of different allelic forms, which are co-dominantly expressed.
What is Graft Rejection?
Rejection of the graft or transplants by the recipient is called as the graft rejection. Graft rejection is mediated by both cell-mediated (T cell) and humoral-mediated (antibodies). Our immune system treats mismatched transplants in the same way as microbes. Thus, if a patient rejects a transplant through transplantation antigens, it will reject a second graft carrying the same or shared antigens much faster.
What is Tissue Typing (Detection of MHC Antigens)?
Tissue typing is the commonly used method for matching donor and recipient for transplantation. In this method, the HLA antigens of class I type on leukocytes are identified by means of antisera obtained from persons who have received multiple blood transfusions and from volunteers who have been immunized with cells having different HLA haplotype. Following methods are used to identify these antigens:
- Cytotoxic Test: Lymphocytes are incubated with a panel of standard sera for HLA antigens (Class I MHC antigens) in the presence of complement. Cells carrying antigens corresponding to the HLA antiserum are killed, which are then detected by the addition of trypan blue (stain only dead cells).
- Mixed Lymphocytes Culture (MLC): This method is used to determine MHC Class II antigens. Lymphocytes from the donor are irradiated or treated with mitomycin C to prevent cell division and then added to cells from the recipient. If the class II antigens on the two cells populations are different, the recipient cells will divide rapidly and take up large quantities of radioactive nucleotides into the newly synthesized nuclear DNA. The amount of radioactive nucleotide can be measured easily.
How can we prevent Graft Rejection during Transplantation?
- Immunosuppression: The immune system of the recipient has to be suppressed to avoid graft rejection. Irradiation, corticosteroids and Anti-Lymphocytic Serum (ALS) are employed for immunosuppression in clinical transplantation. However, some synthetic cytotoxic drugs and some fungal metabolites such as Cyclosporin A and Rapamycin are also used.
- Transplantation in privileged sites: These are certain privileged sites where allografts are permitted to survive. These include the anterior chamber of the eye, cornea, uterus, testis, and brain. Each of these sites is characterized by an absence of lymphatic vessels and sometimes also blood vessels, that means these are areas without significant immune access. Corneal transplantation is the best example of transplantation at such sites.
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