Gallbladder cancer is a rare disease that starts in the cells of the inner layer of the gallbladder. The gallbladder is a small pouch that lies under the liver on the right side of the body. The gallbladder is not a vital organ – the body can function without it. If the gallbladder needs to be removed, there are no serious long-term effects. The most common type of gallbladder cancer is called adenocarcinoma. An adenocarcinoma starts in gland cells that line the gallbladder.