What is the primary function of CR2 on B cells?

What is the primary function of CR2 on B cells?

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

Answer: (B)

CR2 on B cells acts as a co-receptor for antigen by binding the complement fragment C3d that is deposited on antigens during complement activation. When a B cell receptor engages an antigen that also carries C3d, CR2 (as part of the CD19/CD81 co-receptor complex) amplifies the BCR signal. This lowers the activation threshold, making B cells respond more readily, proliferate, and mature into antibody-secreting cells. In short, CR2’s main job is to boost B cell activation by tagging complement-coated antigens and working in concert with the BCR. The other roles listed belong to different components of the complement and immune system: binding C1q to inactivate it is not CR2; increasing immune complex clearance is primarily the job of CR1 on erythrocytes; and binding particles opsonized with C3b is also a function more associated with CR1. CR2 specifically recognizes C3d and serves to enhance B cell responses through co-receptor signaling.

CR2 on B cells acts as a co-receptor for antigen by binding the complement fragment C3d that is deposited on antigens during complement activation. When a B cell receptor engages an antigen that also carries C3d, CR2 (as part of the CD19/CD81 co-receptor complex) amplifies the BCR signal. This lowers the activation threshold, making B cells respond more readily, proliferate, and mature into antibody-secreting cells. In short, CR2’s main job is to boost B cell activation by tagging complement-coated antigens and working in concert with the BCR.

The other roles listed belong to different components of the complement and immune system: binding C1q to inactivate it is not CR2; increasing immune complex clearance is primarily the job of CR1 on erythrocytes; and binding particles opsonized with C3b is also a function more associated with CR1. CR2 specifically recognizes C3d and serves to enhance B cell responses through co-receptor signaling.