After discussing the relevance of statistical physics in molecular recognition processes, we present a schematic model for ligand-receptor association based on an Ising chain. We discuss the possible behaviors of the affinity when the stiffness of the ligand increases. We also consider the case of flexible receptors. A variety of interesting behaviors is obtained, including some affinity modulation upon bond hardening of softening. The affinity of a ligand for its receptor is shown to depend on the details of their rigidity profile, and we question the possibility of encoding information in the rigidities as well as in the shape. An exhaustive study of the selectivity of patterns with length n < 8 is carried on. Connection with other spin models, in particular spin glasses is mentioned in conclusion.