CD22, also known as cluster of differentiation 22, is a 140 kDa type I transmembrane glycoprotein and a member of the Siglec (sialic acid-binding immunoglobulin-like lectin) family. It is predominantly expressed on the surface of mature B cells and to a lesser extent on some immature B cells. The extracellular domain of CD22 consists of seven immunoglobulin-like domains, while its cytoplasmic tail contains conserved tyrosine motifs, including immunoreceptor tyrosine-based inhibitory motifs (ITIMs), which play a crucial role in inhibiting B cell receptor (BCR) signaling. CD22 functions as a negative regulator of B cell activation, helping to prevent overactivation of the immune system and the development of autoimmune diseases. Additionally, it is involved in B cell trafficking and maintaining peripheral B cell tolerance. CD22's interaction with sialic acid-containing molecules on various cell types allows it to modulate B cell responses and maintain immune homeostasis.