Thymoma

Thymoma is a rare cancer that starts in the thymus. The thymus is an organ in your chest, behind the breastbone (the flat bone in the middle of your chest), and its job is to make lymphocytes, white blood cells that help fight infection. Thymoma begins in the thymus's epithelial cells, a type of cell that belongs to this organ. That's why it's called a thymic epithelial tumor.

Even though it's a cancer, its cells are fairly well behaved. They look a lot like normal thymus cells, grow slowly, and rarely spread, meaning they rarely move beyond the thymus.

Here's the important part: thymoma is often linked with autoimmune paraneoplastic diseases. Paraneoplastic means these diseases can show up in people who have cancer. Autoimmune means the immune system, the body's defenses, attacks not just the cancer cells but normal cells too. Picture a security guard who chases the intruder but ends up detaining the neighbors on the block as well. The most common of these diseases is myasthenia gravis, in which the body's defenses attack acetylcholine receptors, the spots on a muscle where the signal from a nerve comes in.