CD22 is a protein present on B cells that regulates their activation. It acts as a natural brake on the B-cell receptor (BCR), helping to prevent excessive immune responses. It is expressed at different stages of B-cell development, but not on plasma cells. It is also involved in other signalling pathways and is internalised by endocytosis, which influences its function. Because of its role in immune regulation, CD22 is an important biomarker and a potential therapeutic target in B-cell diseases.