Tolerance is the lack of an immune response to self. MHC
molecules (gene products from MHC genes) control the body's
ability to recognize self and non-self. When T cells mature
in the thymus, they are checked for their reactivity against
self. Normally all T cells which react against self are destroyed.
Inappropriate T cells produced in the thymus, about 95% of
the total, die and never enter the circulatory system. T cells
in the circulatory system must receive signals from normal
cellular tissue to survive. When these control systems fail,
the immune system can react against our own molecules.
Autoimmunity is an immune response against self (auto=self).Autoimmune
diseases occur when the body attacks its own tissues. Rheumatoid arthritis, myasthenia
gravis and lupus (systemic lupus erythematosus) are examples of autoimmune diseases.