CD21, also known as complement receptor 2 (CR2), is a key membrane protein expressed on the surface of B lymphocytes and follicular dendritic cells. Its core function is to serve as a bridge linking innate and adaptive immunity: on the one hand, as a receptor for complement C3 fragments, it enhances antigen recognition and activation of B cells; on the other hand, it is also the primary receptor for EB virus invasion of B cells and is closely associated with various lymphoproliferative disorders and tumors. Due to its crucial role in immune regulation and pathological processes, CD21 has emerged as a highly regarded potential therapeutic target in the development of drugs for B-cell lymphoma, autoimmune diseases, and anti-EBV infection, holding significant research value particularly in the fields of antibody-based therapeutics and cell therapy.