Compositions, methods and kits for detection of an antigen on a cell and in a biological mixture

An in vitro method of detecting the presence of a moiety bearing an antigen on a cell, said method comprising, a) contacting a cell with a bacteriophage expressing an antibody known to specifically bind with said bearing moiety an antigen, wherein said bacteriophage comprises a nucleic acid and wherein the sequence of said nucleic acid is at least partially known; b) denaturing any said bacteriophage specifically bound with said cell to release said nucleic acid; c) detecting said nucleic acid, wherein detecting said nucleic acid detects the presence of said antigen-bearing moiety on said cell, thereby detecting the presence of said antigen-bearing moiety on said cell.