Complement receptors: Ligands and functions

Receptor	Major ligands	Functions
CR1	C3b/C4b, C1q, MBL, ficolins	Immune adherence; immune complex clearance; regulation of C3b/C4b
CR2	iC3b, C3dg, C3d, IgE receptor, interferon alpha, DNA	B cell coreceptor; immune complex localization; EBV receptor; presents complement-opsonized antigens to T cells
CR3	iC3b, clotting factor X and up to 50 other ligands	Phagocytosis; neutrophil activation; apoptosis; cell activation
CR4	iC3b/fibrinogen/ICAMs	Phagocytosis; cell activation
C3aR	C3a	Cell type dependent; cell activation; histamine release
C4aR	C4a	Cell activation; endothelial cell permeability
C5aR1	C5a	Cell activation; chemotaxis; development and regeneration
C5aR2	C5a	Possible functions include nonsignaling decoy receptor; modulator of C5aR1; G-protein independent signaling
C1qR*	C1q*	Phagocytosis; a variety of C1q receptors have been identified with varied functions*
CRIg	C3b/iC3b	Phagocytosis; alternative pathway inhibitor

CR: complement receptor; MBL: mannose-binding lectin; EBV: Epstein-Barr virus; ICAMs: intercellular cell adhesion molecules; CRIg: complement receptor of the immunoglobulin superfamily.
* Ten putative C1q receptors with diverse structures have been proposed^[1]. All bind C1q as well as other ligands triggering distinct cellular responses. However, these interactions do not lead to complement activation.

References:

Bohlson SS, O'Conner SD, Hulsebus HJ, et al. Complement, C1q, and C1q-related molecules regulate macrophage polarization. Front Immunol 2014; 5:402.

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Complement receptors: Ligands and functions