Which of the following is considered an autoimmune disease?

Rheumatoid arthritis is classified as an autoimmune disease because it occurs when the immune system mistakenly attacks the body's own joint tissues, leading to inflammation and pain. In this condition, the immune response targets synovial membranes, causing swelling and damage to the joints. This autoimmune reaction can ultimately result in loss of function and joint deformities if not managed appropriately.

In contrast, type 2 diabetes is primarily related to insulin resistance and metabolic issues rather than an immune response. Bipolar disorder is a mental health condition characterized by mood swings and is not classified under autoimmune diseases. Hypertension, or high blood pressure, is a cardiovascular condition that involves the effective functioning of the heart and blood vessels, and it is also unrelated to autoimmune processes. Thus, among the provided options, rheumatoid arthritis stands out as the only condition driven by an autoimmune response.