Antibodies, also known as immunoglobulins, share an evolutionarily conserved dimeric core structure with two antigen binding sites. However, recognition of foreign molecules can be achieved by monovalent binding domains, as evidenced by the T-cell antigen receptor and various innate immune receptors. Thus, the reason for the strict evolutionary conservation of immunoglobulin divalence remains unclear. In addition to being soluble immune effector molecules, each immunoglobulin is also expressed as a membrane-bound isoform in the context of the B-cell antigen receptor (BCR). Here, we generated monovalent BCRs and found that their signaling and antigen internalization capabilities were strongly impaired. By using advanced superresolution imaging of BCRs following stimulation with antigens of distinct valences, we showed that the receptor cluster scale in the plasma membrane determines the magnitude of intracellular signaling. The incorporation of additional ITAMs into single BCRs did not increase receptor sensitivity but caused cellular desensitization. Our results demonstrate that the BCR-controlled signaling machinery senses the clustering status of the BCR and that subtle changes in cluster sizes are translated into cellular responses. These findings improve our knowledge of adaptive immune receptor function and will aid in the design of synthetic chimeric antigen receptors.