When normal cells turn into cancer cells, some of the antigens on their surface change. These cells, like many body cells, constantly shed bits of protein from their surface into the circulatory system. Often, tumor antigens are among the shed proteins.

These shed antigens prompt action from immune defenders, including cytotoxic T cells, natural killer cells, and macrophages. According to one theory, patrolling cells of the immune system provide continuous bodywide surveillance, catching and eliminating cells that undergo malignant transformation. Tumors develop when this immune surveillance breaks down or is overwhelmed.