Themes > Science > Life Sciences > General Biology > Immunology > The Complement System > Complement Receptors

CR1 binds C3b and C4b. CR1 is found on erythrocytes, where it plays a vital role in removing immune complexes from circulation (see above). It is also found on macrophages and neutrophils and can trigger phagocytosis (only after activation of cells by other mediators including C5a). CR1 also has a regulatory role (see above).

CR2 binds iC3b and C3d and is found on B cells where it plays an important role in activating class switching and memory formation.

Added Value
The presence of bound C3d molecules enhances the response to antigen by about 20-fold for each molecule of C3d bound (at least up to 3). In this way the innate immune system guides the antibody response onto the most 'dangerous' antigens and greatly lowers the threshold of antigen required to generate a response. This effect is mediated by CR2 crosslinking with surface IgM. CR2 recruits a signalling chain which both amplifies signals and triggers a distinct signalling pathway.

CR3 and CR4 are related receptors which bind iC3b and are found on monocytes/macrophages and neutrophils. They trigger phagocytosis of opsonised particles, either in concert with Fc receptors or independently. Phagocytosis of microorganisms via CR3, and via CR1+FcR, is the major defense mechanism against bacterial and fungal infection.


Information provided by: http://www-immuno.path.cam.ac.uk