Final answer:
Autoimmune diseases are characterized by a loss of self-tolerance, where the immune system erroneously attacks the body's own tissues. This commonality is influenced by genetic predisposition, environmental triggers, and the immune system's regulatory mechanisms.
Step-by-step explanation:
The single factor that all autoimmune diseases have in common is loss of self-tolerance. Autoimmune diseases occur when the immune system mistakenly attacks the body's own cells, conditions where immune tolerance is breached, leading to an immunologic attack on one's own tissues. Factors contributing to this loss of tolerance include genetic predisposition, environmental triggers and defects in regulatory T cell functions.
While certain genetic factors such as MHC gene expression can predispose individuals to autoimmune diseases, they only manifest if self-tolerance is compromised. Environmental factors, such as infections, may also precipitate an autoimmune response. Furthermore, regulatory T cells usually inhibit autoreactive T cells, and a defect here can result in autoimmunity.
Therefore, while genetic predisposition, antibody production, and viral infections can influence the development of an autoimmune disease, the core pathology across these diseases is a loss of self-tolerance.