•Application of strict diagnostic criteria to celiac disease has led to the realization that there is a wide clinical spectrum within this disease.•Normally there are immune reactions to dietary proteins, particularly secretory antibodies of IgA class, low titers of serum antibody and specific down‐regulation of IgE and T cell reactivity (oral tolerance).•From time to time, abnormal immunity to foods, either as inappropriately high titers of antibodies or qualitatively altered responses, produce disease.•Lactase deficiency, with dose‐related lactose and milk‐intolerance, occurs in 50–90% of most populations. White, western Europeans are the exception.