The thymus gland, which lies in the upper chest area beneath the breastbone, plays an important role in the development of the immune system in early life. Its cells form a part of the body"s normal immune system. The gland is somewhat large in infants, grows gradually until puberty, and then gets smaller and is replaced by fat with age. In adults with myasthenia gravis, the thymus gland is abnormal. It contains certain clusters of immune cells indicative of lymphoid hyperplasia-a condition usually found only in the spleen and lymph nodes during an active immune response. Some individuals with myasthenia gravis develop thymomas, or tumors of the thymus gland. Generally, thymomas are benign, but they can become malignant.
The relationship between the thymus gland and myasthenia gravis is not yet fully understood. Scientists believe the thymus gland may give incorrect instructions to developing immune cells, ultimately resulting in autoimmunity and the production of acetylcholine receptor antibodies, thereby setting the stage for the attack on neuromuscular transmission.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.