You need a source of HLA to do the initial virtual cross match (detection of HLA specific antibodies). This is accessed from donor WBCs. The HLA profile obtained is then compared against recipient profiles for known anti-HLA. If no known antibodies are present against the HLA profile obtained, there is a very low chance of rejection. In some cases (highly sensitized patients) a cross match on the day of transplant is repeated to ensure that no antibodies developed in the interim. This is called a "STAT cross match"
A nice summary of the luminex principle is included below.
As a clarification Luminex is done to generate the HLA antibody profile for recipients prior to transplant using stock HLA, not those of the donor. These profiles are generated every few months with serum provided by the recipients.