Complement receptor

A complement receptor is a receptor of the complement system, part of the innate immune system. Complement receptors bind proteins of the complement system, and can thus detect pathogens without mediation by antibodies. Complement activity can be triggered by specific antigen-antibody complexes, sugars or pathogens directly. Many white blood cells express complement receptors on their surface, particularly monocytes and macrophages. All four complement receptors bind to fragments of complement component 3 or complement component 4 coated on pathogen surface, but the receptors have different functions. Complement receptor (CR) 1, 3, and 4 work as opsonins (stimulate phagocytosis), whereas CR2 is expressed only on B cells as a co-receptor. Red blood cells (RBCs) also express CR1. With these receptors, RBCs bring antigen-antibody complexes bound to complement fragments in the blood to the liver and spleen for degradation. Defects in these receptors can be associated with disease.