Four units of blood are ordered to be crossmatched on a 78 y…

Four units of blood are ordered to be crossmatched on a 78 year old who is A Rh Positive with a history of a previous anti Jka. Today, the antibody screen is negative. Three of the 4 units selected for crossmatch are incompatible when tested at AHG phase. The most likely antibody causing this incompatible reaction is: