Immunologic rejection is mediated by the recipient's:
A. EosinophilsTransplants between genetically nonidentical persons lead to recognition and rejection of the organ by the recipient's immune system, if no intervention is undertaken. The main antigens responsible for this process are part of the major histocompatibility complex (MHC). In humans, these antigens make up the HLA system. The antigen-encoding genes are located on chromosome 6. Two major classes of HLAs are recognized. They differ in their structure, function, and tissue distribution. Class I antigens (HLA-A, HLA-B, and HLA-C) are expressed by all nucleated cells. Class II antigens (HLA-DR, HLA-DP, and HLA-DQ) are expressed by antigen presenting cells (APCs) such as B lymphocytes, dendritic cells, macrophages, and other phagocytic cells. The principal function of HLAs is to present the fragments of foreign proteins to T lymphocytes. This leads to recognition and elimination of the foreign antigen with great specificity. HLA molecules play a crucial role in transplant recipients as well. They can trigger rejection of a graft via two different mechanisms. The most common mechanism is cellular rejection, in which the damage is done by activated T lymphocytes.