What is a protein that is able to distinguish between a foreign protein or cell?

MHC Protein
The major histocompatibility complex (MHC) is a set of cell surface proteins essential for the acquired immune system to recognize foreign molecules in vertebrates, which in turn determines histocompatibility. The main function of MHC molecules is to bind to peptide fragments derived from pathogens and display them on the cell surface for recognition by the appropriate T-cells.[1] MHC molecules mediate interactions of leukocytes, also called white blood cells (WBCs), which are immune cells, with other leukocytes or with body cells. The MHC determines compatibility of donors for organ transplant, as well as one's susceptibility to an autoimmune disease via cross reacting immunization. In humans, the MHC is also called the human leukocyte antigen (HLA).

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