Human Leukocyte Antigens (HLA) are a set of highly polymorphic cell surface proteins encoded by genes within the Major Histocompatibility Complex (MHC) on chromosome 6. Their specific immunological role in determining graft compatibility stems from their primary function: presenting peptide antigens to T lymphocytes, thereby enabling the immune system to distinguish "self" from "non-self." There are two main classes relevant to transplantation: HLA Class I (HLA-A, -B, -C), found on nearly all nucleated cells, and HLA Class II (HLA-DR, -DQ, -DP), primarily expressed on professional antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. HLA Class I molecules present endogenous peptides (from inside the cell) to CD8+ cytotoxic T lymphocytes, while HLA Class II molecules present exogenous peptides (from outside the cell) to CD4+ helper T lymphocytes.
In the context of graft compatibility, HLA molecules are the primary targets of the recipie....
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