Abstract: The present invention relates to novel methods for detecting at least one member of a known binding pair in a sample, including a cell, where one member of the pair (termed the “receptor”) is expressed by a bacteriophage, which phage is then used to detect the presence of the other member of the pair (termed the “ligand” or “target”). Rather than detecting the binding of at least one phage using antibody-based technology, the present invention relates to detecting the nucleic acid associated with the phages. In one aspect, the invention relates to identifying at least one antigen-bearing moiety (e.g., a red blood cell antigen) of interest present on a cell, e.g., a red blood cell, using antibody-displaying bacteriophages, using antiglobulin reagent-displaying bacteriophages and detecting at least one nucleic acid associated with the phage.