For most important antigens of diagnostic significance, antisera are commercially available. The most common test methods for bacteria are agglutination and immunofluores-cence; and for viruses, neutralization. In most cases these methods subclassify organisms below the species level and thus are primarily of value for epidemiologic and research purposes. The terms “serotype” or “serogroup” are used together with numbers, letters, or Roman numerals with no apparent logic other than historical precedent. For a few genera the most fundamental taxonomic differentiation is serologic. This is the case with the streptococci , where an existing classification based on biochemical and cultural characteristics was superseded because a serologic classification scheme devel-oped by Rebecca Lancefield correlated better with disease.
Before these techniques can be applied to the diagnosis of specific infectious diseases, considerable study of the causative agent(s) is required. Antigen – antibody systems may vary in complexity from a single epitope to scores of epitopes on several macromolecular antigens whose chemical nature may or may not be known. The cause of the original 1976 outbreak of Legionnaires’ disease (caused by Legionella pneumophila) was proven through the development of immune reagents that detected the bacteria in tis-sue and antibodies directed against the bacteria in the serum of patients. Now, more than 25 years later, there are more than a dozen serotypes and many additional species, each re-quiring specific immunologic reagents for antigen or antibody detection for diagnosis.