Abnormality of immune system physiology Abnormal MHC surface expression 2025-03-01T11:10:50Z A deviation from the normal level of major histocompatibility complex (MHC) molecules expressed at the cell surface. The cell-mediated adaptive immune response is regulated by the MHC, which is called human leukocyte antigen (HLA) in humans. HLA molecules are cell surface glycoproteins whose primary function is to present endogenous and exogenous antigens to T lymphocytes for recognition and response. The HLA molecules that present antigen to T lymphocytes are divided into two main classes: HLA class I and HLA class II molecules.