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Complement Receptor 2 (CR2), also known as CD21, is a protein that in humans is encoded by the CR2 gene. CR2 is a member of the complement system, which is part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen's cell membrane. It is involved in the immune response and is a receptor for C3d, C3dg, and iC3b complement components, as well as for the Epstein-Barr virus (EBV).

Function

CR2 is primarily expressed on mature B cells, follicular dendritic cells, and a subset of T cells within the human immune system. It plays a critical role in the B cell activation and adaptive immune response by binding to C3 fragments that are attached to antigens, promoting B cell activation and antigen presentation. This interaction is crucial for the initiation of the immune response and the formation of immunological memory.

Structure

The CR2 protein is characterized by a series of short consensus repeats (SCRs) also known as Sushi domains. These domains are involved in the binding of complement fragments and possibly other ligands. The gene encoding CR2 is located on chromosome 1 in humans and contains 19 exons.

Clinical Significance

Alterations in the expression or function of CR2/CD21 have been associated with a variety of autoimmune diseases, including systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). Additionally, since CR2 is a receptor for the Epstein-Barr virus, it plays a role in the initial steps of EBV infection, making it a subject of interest in studies on infectious mononucleosis and EBV-associated malignancies.

Research

Research on CR2 has focused on its role in the immune system, its potential as a therapeutic target in autoimmune diseases and infectious diseases, and its involvement in vaccine development. Understanding the mechanisms of CR2-mediated B cell activation and its interactions with other components of the immune system could lead to new therapeutic strategies for managing immune-related diseases.