CD21 (SP199)

CD21 antigen, also known as complement receptor type 2 (CR2), is a glycoprotein and a receptor for the C3d fragment of complement component 3. It is predominantly expressed in follicular dendritic cells and B lymphocytes. CD21 binds to CD19, and CD81 to form a B cell complex on mature B cells and enhances the B cell response to antigen through C3d.

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