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Why can autoimmune diseases run in families?

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immune system disorder False-color scanning electron micrograph of a T cell infected with HIV (human immunodeficiency virus), the agent that causes AIDS (acquired immunodeficiency syndrome).

Autoimmune diseases often run in families because they are linked to a genetic predisposition passed from one generation to the next. Studies comparing identical and fraternal twins confirm that this increased incidence is not merely the result of shared environmental factors, but stems from inherited genetic defects.12

Rather than being caused by a single mutation, researchers generally agree that the development of these conditions involves the interaction of multiple genes. For instance, type 1 diabetes mellitus is believed to result from the influence of at least 14 different genes. Individuals who possess specific MHC antigens on their cells are more likely to develop certain autoimmune disorders, though possessing these markers does not guarantee that a person will contract the disease.2

While genetics play a significant role, most human diseases—including those with a familial tendency—arise from a complex interplay between an individual's genetic constitution and environmental influences. Factors such as infections, chemical exposure, and hormonal effects may also trigger or exacerbate these conditions in genetically susceptible individuals.3124