CR2 (Complement Receptor type 2), also known as CD21. is a transmembrane glycoprotein primarily expressed on B lymphocytes, follicular dendritic cells, and some epithelial cells. It plays a critical role in the immune system by binding complement fragments such as C3d, which opsonize pathogens or immune complexes. This interaction enhances B cell activation by synergizing with the B cell receptor (BCR), lowering the threshold for antigen recognition and promoting adaptive immune responses. CR2 also serves as a receptor for Epstein-Barr virus (EBV), facilitating viral entry into B cells.
Structurally, CR2 consists of 15 or 16 short consensus repeat (SCR) domains, with the C3d-binding site located in SCR1-2. Its involvement in both innate and adaptive immunity makes it a key player in immune regulation and pathogen defense. Dysregulation of CR2 has been implicated in autoimmune diseases (e.g., lupus), B cell malignancies, and chronic infections.
CR2-targeting antibodies are valuable tools for studying B cell biology, immune complex clearance, and viral entry mechanisms. Therapeutic applications include developing inhibitors for EBV infection, modulating B cell activity in autoimmunity, and designing antibody-drug conjugates for targeted B cell therapies. Research continues to explore its dual role as a immune enhancer and pathogenic gateway, balancing therapeutic potential with safety considerations.