Siglec-2, also known as CD22, is a sialic acid-binding immunoglobulin-type lectin predominantly expressed on B cells in humans and other mammals. It is a 140 kDa glycoprotein with seven Ig-like domains and multiple intracellular ITIM motifs, showing high selectivity for α2-6 linked sialic acids. CD22 spontaneously associates with the B cell receptor （BCR） in a sialic acid-independent manner and down-regulates B cell activation. The binding of CD22 to its sialoglycan ligands can either activate or suppress B cell function, depending on the location of the ligands. For instance, cis-ligation on the same cell membrane enhances B cell activation by sequestering CD22 away from the BCR, while trans-ligation with sialoglycans on antigen-presenting cells can inhibit the BCR. CD22 is heavily glycosylated, containing 11 potential N-glycosylation sites, six of which are in the N-terminal two domains. It is also involved in various biological processes, such as B cell homing and adhesion to other immune cells. Additionally, CD22 is a target receptor in anti-cancer therapies for lymphoma and leukemia, as well as in the treatment of autoimmune diseases like lupus and rheumatoid arthritis.