What does CR2 bind to in the context of the immune response?

CR2, also known as CD21, plays a critical role in the immune response by functioning as a receptor for complement components. Specifically, CR2 binds to C3d, which is a degradation product of the complement component C3.

In the context of the immune response, the binding of CR2 to C3d enhances B cell activation and proliferation upon encounter with antigens that have been opsonized, meaning that they have been marked for destruction by immune cells. The interaction between CR2 and C3d provides a necessary signal that promotes B cell responses, such as class switching and memory formation, thereby facilitating the development of effective long-term immunity.

This connection between CR2 and C3d is significant because it highlights how the complement system assists in adapting the immune response, not just through direct pathogen destruction but also through enhancing B cell activity. This is crucial during the formation of germinal centers where B cells actively proliferate and differentiate into memory B cells and plasma cells that produce antibodies.