Determining ABH secretor phenotype and/or Lewis (Le) blood group status can be useful to the metabolically-oriented clinician. For example, differences in ABH secretor status drastically alter the carbohydrates present in body fluids and secretions; this can have profound influence on microbial attachment and persistence. Lewis typing is one genetic marker which might help identify subpopulations of individuals genetically prone to insulin resistance, autoimmunity, and heart disease. Understanding the clinical significance of ABH secretor status and the Lewis blood groups can provide insight into seemingly unrelated aspects of physiology, including variations in intestinal alkaline phosphatase activity, propensities toward blood clotting, reliability of some tumor markers, the composition of breast milk, and several generalized aspects of the immune function. Since the relevance of ABH blood group antigens as tumor markers and parasitic/bacterial/viral receptors and their association with immunologically important proteins is now well established, the prime biologic role for ABH blood group antigens may well be independent and unrelated to the erythrocyte.