Disclosed are methods for detecting antibody in a sample, where the antibody targets an antigen expressed by red blood cells or red blood cell ghosts. Rather than detecting the binding events between a particular antigen antibody pair (as in traditional agglutination based assays) the methods herein allow for multiplexed detection of clinically important allo-immune antibodies to blood group antigens. Specifically the method involves generating fluorescently encoded red blood cells or red blood cell ghosts with known antigen presentation and using them to detect the presence of antibody in serum/plasma with a fluorescent sandwich type immunoassay. The assay results can be read using flow cytometric or fluorescent microscope based imaging techniques.