Alternatively, the ligand- or ligand binding unit can be bound to a bridging member either covalently or immunologically (herein designated by ???*???) For example, the ligand- or ligand binding unit can be covalently bound to a linking agent such as glutaraldehyde which in turn is covalently bound to a proteinaceous bridging member such as bovine serum albumin (BSA) which in turn is adsorbed on t