Abstract: A library of isolated and purified antigens specific for a microorganism is a set of individual molecules. The library forms antigen-antibody complexes useful in the context of diagnosing and treating conditions associated with a specific microorganism such as H. pylori-induced gastro-duodenal disease. For the antigen-antibody complexes in question the antibody is an immunoglobulin, which is IgE if the antigens are allergens. Complexes with IgA, IgG and IgM are also useful. By this multivariate approach, a specific condition is diagnosed with high sensitivity and specificity by determining whether complexes form between a specific antigen library and a biological sample which contains immunoglobulins from an individual. Such libraries also are useful for immunotherapy.