An autoimmune disease occurs when the immune system mistakenly identifies the body's own cells as foreign and mounts an attack against them. This aberrant immune response leads to inflammation and tissue damage in various organs. Unlike infections or allergies, autoimmune diseases are characterized by self-directed immunity. Understanding this mechanism is critical for diagnosis and treatment, as therapies often aim to suppress the immune response to protect normal tissues.

Reference: Robbins Basic Pathology, Kumar et al., 10th Edition.