In addition to what has been nicely explained by Malcolm, it could be as well an example of Sd(a++) cell (commonly named "super Sid") reacting with a weak anti-Sda. The Sda antigen is not a LFA (expressed on more than 90% of cells) though some cells "overexpresse" it.
Anti-Sda usually gives weak/DP reactions and can be neutralized using urine (contains soluble Sda substances).
Other weak antibodies may behave the same way, e.g. anti-P1 reacting against "strong P1" cells only.
However, that does not change at all what Malcolm said "I wouldn't expend too much time or energy trying to sort out the exact specificity. In all cases of such an antibody, as long as you cross-match by the same method as you used in detecting the presence of the antibody in the first place, it would be quite safe to give cross-match compatible blood."