In a direct (forward) donor-recipient crossmatch, which reagents are used?

• A. Donor serum and recipient lymphocytes + rabbit serum complement
• B. Recipient stimulator cells + donor responder cells + complete culture medium
• C. Donor stimulator cells + recipient responder cells + complete culture medium
• D. Recipient serum and donor lymphocytes + rabbit serum complement

Answer: (D)

The essential idea is to detect whether the recipient has antibodies that recognize the donor’s cells, by testing antibody binding with a cytotoxic readout. In a direct (forward) crossmatch, you use the recipient’s serum as the source of antibodies and combine it with donor lymphocytes as the target cells carrying the donor antigens. Adding rabbit serum complement allows any antibodies that bind to the donor cell antigens to fix complement and cause lysis of the donor cells. A positive lysis result means donor-specific antibodies are present and compatibility is limited; a negative result suggests no such antibodies.

Thinking about the setup in other terms helps: using donor serum with recipient cells would be the reverse/indirect approach, not the forward crossmatch, and using only culture medium wouldn’t test for antibody-mediated cytotoxicity.