Which statement best describes the role of CR2 on cell membranes?

Which statement best describes the role of CR2 on cell membranes?

• A. Binds C1q to inactivate it
• B. Increases clearance of immune complexes
• C. Acts as co-receptor on B cells for antigen
• D. Binds particles opsonized with C3b

Answer: (C)

CR2 (CD21) on B cells serves as a co-receptor by recognizing the complement fragment C3d that is bound to an antigen. When a B cell receptor binds its antigen, if that antigen is tagged with C3d, CR2 engages C3d and works together with CD19 (and CD81) to amplify the B cell’s activation signal. This lowers the threshold for activation and enhances the antibody response, especially for antigens that have been opsonized by complement. This is why the statement describing CR2 as a co-receptor on B cells for antigen is the best fit. It isn’t primarily involved in binding C1q to inactivate it, nor in clearing immune complexes (that role is more associated with CR1). It also doesn’t bind particles opsonized with C3b—CR2 specifically recognizes C3d, the fragment that remains attached to antigen and helps boost B cell signaling.

CR2 (CD21) on B cells serves as a co-receptor by recognizing the complement fragment C3d that is bound to an antigen. When a B cell receptor binds its antigen, if that antigen is tagged with C3d, CR2 engages C3d and works together with CD19 (and CD81) to amplify the B cell’s activation signal. This lowers the threshold for activation and enhances the antibody response, especially for antigens that have been opsonized by complement.

This is why the statement describing CR2 as a co-receptor on B cells for antigen is the best fit. It isn’t primarily involved in binding C1q to inactivate it, nor in clearing immune complexes (that role is more associated with CR1). It also doesn’t bind particles opsonized with C3b—CR2 specifically recognizes C3d, the fragment that remains attached to antigen and helps boost B cell signaling.