1). tumor-specific antigen. tumor-specific antigen (TSA) any cell-surface antigen of a tumor that does not occur on normal cells of the same origin. V antigen (Vi antigen) an antigen contained in the sheath of a bacterium, as Salmonella typhosa (the typhoid bacillus), and thought to contribute to its virulence.. any of several antigens, coded for by the viral genome, associated with transformation of infected cells by certain DNA tumor viruses. Called also tumor antigen. 2. an antigen present on human erythrocytes that is exposed by treatment with neuraminidase or contact with certain bacteria.