The immune system's ability to distinguish self from nonself is essential for both host defence
against microbial antigens and protection of self-antigens from autoimmune destruction.
Such discrimination is complicated by extensive structural homology shared between micro-
organisms and self-antigens, a condition known as molecular mimicry. Molecular mimicry
provides the foundation for an immune response directed against an exogenous agent such
as a virus to cross-react with mimicked host self sequences, leading to autoimmunity, and in …