the present invention related to heterobifunctional cross - linkers substituted by a cell surface binding group or a cell recognising group , and also by a bio - active group , the bio - active group attached to a carbonyl of the cross - linker by an acid labile bond . examples of suitable bio - active groups include an immunomodulator and an anti - neoplastic group . the conjugate enables , for example , the coupling of immunomodulators ex vivo to a target cell , for example a tumor antigen presenting cell . the conjugate can act as a non - toxic prodrug carrying a cytotoxin , for example , to a target tissue site , where the ph - dependency of the labile amide bond provides for cell selectivity of the masked cytotoxin . the invention further relates to a method for producing the cross - linkers , their use in treating disease , and ex - vivo cell labeling .