What is the significance of DR antigen in the HLA system?

The DR antigen is part of the HLA (Human Leukocyte Antigen) system, specifically categorized under class II antigens. Its primary significance lies in organ transplantation, where matching HLA antigens between donor and recipient can play a crucial role in the success of the transplant procedure. A close match of these antigens helps minimize the risk of transplant rejection by the recipient's immune system, as it recognizes the donor tissue as foreign.

In the context of organ transplantation, mismatched DR antigens can lead to the activation of T cells that attack the transplanted organ, making it vital for transplant centers to perform HLA typing. Matching the DR antigen in addition to other HLA class I and class II antigens helps in improving graft survival rates and reducing complications during the post-transplant period.

Other options regarding the DR antigen are less relevant. For instance, while DR antigens can be identified through various testing methods, they are not expressed on platelets or granulocytes in a manner that is significant to their role in organ transplantation. Additionally, DR antigens are indeed detectable in lymphocytotoxicity tests, countering the assertion that they are not detectable.