According to ASHI standards (2005), how many sera must be used to define a class I antigen by microcytotoxicity?

Defining a class I antigen by microcytotoxicity testing requires the use of two operationally monospecific antisera or monoclonal antibodies. This is because the use of two different, carefully selected antisera allows for the precise identification of the specific antigen being tested. Monospecific antisera recognize only one epitope of the antigen, which ensures that the results are not confounded by cross-reactivity with other antigens. This is critical in histocompatibility testing for organ transplantation, where accurate antigen matching can influence graft survival and patient outcomes.

Operationally monospecific refers to sera that have been validated to target specific antigens without interference from others, making them ideal for testing. In addition, using two antisera can help to confirm the presence or absence of an antigen, adding an extra layer of validation to the results. Other options may suggest different minimum numbers of sera or antigens, which do not align with the ASHI standard requirements, thus reinforcing the choice of two operationally monospecific sera or monoclonal antibodies as the correct answer for accurately defining class I antigens.