Antigenic mimicry and autoimmune diseases. 
The finding of cross-reactive autoantibodies or sequence homology does not necessarily mean that this molecular mimicry is biologically meaningful or associated with disease pathogenesis.  For example, relatives of persons with putative autoimmune insulin-dependent diabetes [123], and elderly humans [124] have a high incidence of autoantibodies which are generally not associated with autoimmune disease.  In addition, natural antibodies to cell constituents [125] may be present in normal sera.  These antibodies need to be directed against biologically important domains of host cell proteins in order to mediate autoimmune disease [27].  In spite of extensive homology between two sequences, a cross-reactive immune response may not be generated.  The dissimilar amino acids should not be radical substitutions or affect the binding properties of the molecule.  For instance, antibodies to synthetic peptides with only one substitution in a 19 amino acid sequence may not bind the whole protein [126].  Despite an identical six amino acid sequence shared by HLA-B27 and an EBV protein, no cross-reactive antibodies to EBV peptides were found in HLA-B27 positive patients with AS or RS.  Unless the homology and subsequent crossreactive immune response can recognize a host protein intimately involved in disease pathogenesis, autoimmune disease is unlikely to occur.