Human leukocyte antigen (HLA) is a protein found in our body that is used to match us with a donor for bone marrow or cord blood transplant. The HLA system is a gene complex coding the major histocompatibility complex (MHC) proteins. These cell-surface proteins control the immune system in people. The HLA complex dwells on a 3 Mbp stretch inside chromosome 6p21 and is polymorphic, which implies that they have various alleles, enabling them to tune the adaptive immunity. The proteins coded by specific genes are called antigens, because of their noteworthy revelation as elements in organ transplants. HLAs relating to MHC class I (A, B, and C) exhibit peptides from inside the cell. Remote antigens exhibited by MHC class I pull in killer T-cells (additionally called CD8 positive-or cytotoxic T-cells) that obliterate cells. MHC class I proteins connect with β2-microglobulin, which not at all like the HLA proteins is coded by a gene on chromosome 15. HLAs relating to MHC class II (DP, DM, DOA, DOB, DQ, and DR) display antigens from outside of the cell to T-lymphocytes. These specific antigens incite the duplication of T-helper cells, which thus invigorate antibody delivering B-cells to create antibodies to that particular antigen. Self-antigens are smothered by regulatory T cells. HLAs symbolizing to MHC class III code segments of the complement system.