The paradoxical relationship between immunodeficiency (under-responsiveness) and autoimmunity (over-responsiveness) affecting the immune system was debated at the Fourth AARDA Colloquium on cross-disciplinary issues in autoimmunity. Immunodeficiency disease and autoimmune disease, far from being mutually exclusive, share profound dysregulation of the immune system. Among the most keenly discussed issues were: i) the remarkably high number of molecularly identified primary immunodeficiencies with autoimmune expressions; ii) the homeostasis of immune function such that deficiency in any one given compartment can result in over activity in the same or another compartment; iii) whilst some immune deficiency states are essentially monogenic, each of them can exhibit striking variability in autoimmune outcome, indicative of epigenetic or environmental influences on phenotypic expression; iv) innate immunity, particularly complement defects, as well as adaptive immunity, is complicit in the immunodeficiency-autoimmunity axis; v) features of certain of the disorders discussed at the meeting forced a reappraisal of what actually is meant by 'autoimmune disease'. It was concluded that genes that determine inherited immunodeficiencies, hitherto rather neglected by autoimmunologists, compel attention to consideration of molecular genetic anomalies critical for emergence of autoimmune disease in humans and animals.

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