The immune system must tell apart the body's own cells (self) from anything foreign (non-self). It does this by recognising antigens.An antigen is a molecule, usually a protein or glycoprotein, that can trigger an immune response. Antigens are found on the surfaces of cells and viruses, and toxins can also act as antigens. Each antigen has a specific shape, so it can be recognised by receptors with a complementary shape.Every nucleated body cell carries self antigens, including proteins of the major histocompatibility complex (MHC). The combination of MHC proteins is almost unique to each individual (except identical twins). During development, lymphocytes with receptors that bind strongly to self antigens are destroyed, so the immune system normally tolerates the body's own cells.Anything with non-self antigens is recognised as foreign and can be attacked, including:
• pathogens such as bacteria, viruses, fungi and protozoa
• toxins released by pathogens
• cells from other individuals, as in transplanted organs or mismatched blood
• abnormal body cells, such as cancer cells or virus-infected cells, which display altered or foreign antigens.Body cells also use MHC proteins to display fragments of molecules from inside them. A cell infected by a virus displays viral antigen fragments on its MHC proteins, signalling to T cells that it is infected.This explains transplant rejection: the recipient's immune system recognises the donor's MHC proteins as non-self. Doctors match tissue types closely and give immunosuppressant drugs to reduce the risk.
• pathogens such as bacteria, viruses, fungi and protozoa
• toxins released by pathogens
• cells from other individuals, as in transplanted organs or mismatched blood
• abnormal body cells, such as cancer cells or virus-infected cells, which display altered or foreign antigens.Body cells also use MHC proteins to display fragments of molecules from inside them. A cell infected by a virus displays viral antigen fragments on its MHC proteins, signalling to T cells that it is infected.This explains transplant rejection: the recipient's immune system recognises the donor's MHC proteins as non-self. Doctors match tissue types closely and give immunosuppressant drugs to reduce the risk.