Table B.

The major histocompatibility complex (MHC)

The major histocompatibility complex (MHC) is a group of genes that code for proteins found on the surfaces of cells that help the immune system recognize foreign substances; more preciously pathogen-derived peptides bound to MHC molecules. In humans, the complex is also referred to human leukocyte antigen (HLA). MHC genes are grouped into class I, class II, and class III depending on their location on the respective chromosome, structure, and function. MHC class I molecules, which are present on all nucleated cells 152, are also functional in the innate immune system by acting as ligands of inhibitory killer cell immunoglobulin-like receptors (KIRs) on natural killer (NK) cells. NK cells play a key role because they have the unique ability to recognize and non-specifically destroy cells lacking self MHC class I molecules. Because all (healthy) nucleated cells express self MHC class I molecules, inhibitory KIRs ensure that NK cells are not attack normal cells but they kill infected and tumour cells, which downregulate MHC molecules 153.