T-antigen, also known as Tumor antigen, refers to a type of protein produced by a virus or a mutated cell that can induce an immune response in the host organism. T-antigens can be found on the surface of cancer cells or in viral-infected cells and are often used by the immune system to identify and target these abnormal cells for destruction.

In the context of Simian Virus 40 (SV40), T-antigen refers to a viral protein that plays a critical role in viral replication and transformation of the infected host cells. SV40 large T-antigen (often abbreviated as LT) is a multifunctional protein that interacts with cellular proteins and influences cellular processes such as DNA replication, cell cycle progression, and cellular transformation.

T-antigens are also used in cancer research and immunotherapy. By studying these antigens, scientists can develop a better understanding of the mechanisms involved in cancer development, and potentially design targeted therapies to specifically attack cancer cells expressing these antigens, while sparing healthy cells.