The Major Histocompatibility Complex (HLA)

Key points

• • The major histocompatibility complex (MHC), called the HLA system in humans, is a group of cell-surface molecules that hold up protein fragments (antigens) for T cells to inspect.
• • There are two classes: MHC class I is on almost all cells and shows fragments to cytotoxic T cells, while MHC class II is on immune cells and shows fragments to helper T cells.
• • This display lets the immune system tell self from non-self, so that infected or foreign cells are recognised and dealt with.
• • The HLA genes are among the most polymorphic in the human genome, so two unrelated people rarely have the same set, which is the basis of tissue matching for transplants.
• • The closer the HLA match between donor and recipient, the lower the risk of rejection, though rejection also depends on many other factors.
• • Certain HLA types are linked with particular diseases; for example HLA-B27 is strongly associated with the spinal arthritis ankylosing spondylitis.