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Siglec-2, also known as CD22, is a type I transmembrane protein belonging to the immunoglobulin superfamily and functions as an inhibitory lectin on B cells. It is characterized by its ability to bind α2,6-linked sialic acid on the same （cis） or adjacent （trans） cells. CD22 plays a crucial role in mediating B cell interactions, directing B cell localization within lymphoid tissues, and regulating B cell antigen receptor signaling. The protein contains four immunoreceptor tyrosine-based inhibitory motifs （ITIMs）, allowing it to recruit cytoplasmic phosphatases to inhibit downstream signaling pathways. Additionally, CD22 can form cis interactions with sialylated glycoproteins such as CD45, potentially masking its sialic acid recognition site. Its complex interactions with various signaling molecules highlight its multifaceted role in B cell regulation and immune responses.