What system is referred to as the human leukocyte antigen (HLA) system in humans?

The human leukocyte antigen (HLA) system is synonymous with the major histocompatibility complex (MHC) in humans. This complex plays a crucial role in the immune response by presenting peptide fragments from pathogens and other molecules to T cells, which are essential for the adaptive immune system. The HLA molecules are highly polymorphic, meaning there is a vast diversity of these antigens within the human population. This diversity is critical for the immune system's ability to recognize a wide array of foreign antigens.

The MHC is divided into two main classes: Class I, which presents antigens to CD8+ cytotoxic T cells, and Class II, which presents antigens to CD4+ helper T cells. This differentiation is vital for initiating appropriate immune responses, allowing the body to target and eliminate infected or malignant cells. Understanding the HLA system is also fundamental in fields such as organ transplantation and autoimmune diseases, as incompatible HLA types can lead to transplant rejection or dysregulated immune responses.