Helicobacter pylori infection is implicated in the pathogenesis of extradigestive diseases such as acne rosacea and idiopathic chronic urticaria and autoimmune diseases such as autoimmune gastric atrophy, rheumatoid arthritis, anti phospholipid antibody syndrome, autoimmune thyroiditis, Sjoegren syndrome, Henoch-Schoenlein purpura, and Type B insulin resistance syndrome. H. pylori eradication ameliorated the condition in some, but not all, of those with these autoimmune diseases. Recent studies primarily in Italy and Japan found that H. pylori eradication in those infected with chronic immune thrombocytopenic purpura (ITP) results in a persistent platelet count increase in over half of those treated, suggesting that although pathogenetic mechanisms underlying the relationship between H. pylori infection and autoimmune disease remain unclear, yet-unknown immunological events induced by H. pylori infection almost certainly occur in the development of autoimmune response. A majority of isolated H. pylori strains express human Lewis (Le(x) and/or Le(y) determinants and in some strains, Le(a), Le(b), sialyl-Le(x)), and H determinants in the O-chain of the surface lipopolysaccharide. Previous studies showed that this molecular mimicry helps the bacterium evade host responses while evoking autoantibody responses to Le antigens. The anti-Le(y) autoantibody is also reported to promote H. pylori adhesion to gastric epithelial cells, leading to development of gastric atrophy. Moreover, one can hypothesize that anti-Le autoreactive antibodies induced by H. pylori infection are involved in the development of autoimmune diseases, although no clinical studies showing that anti-Le immune responses are involved in the etiology of these autoimmune diseases have been conducted. Proving this hypothesis would require quantitative and qualitative analysis of autoantibodies and T cell functions to Le antigens. High frequent phase variation of Le structures in the O-polysaccharide of H. pylori may influence the immune response of patients to Le antigens.