The innate immune system identifies pathogens by recognizing known elements. This response ultimately leads to the production of proinflammatory substances and the activation of phagocytic neutrophils and macrophages¹. Activation of the complement system is an important initial even in the response against pathogens. This system includes some thirty different proteins that act together to form a complex¹.

Three different pathways are known to activate the complement. The C3 complement plays a key role in all three pathways leading to the activation of the complement. Produced in the liver, C3 is the most abundant complement protein in the serum. It is composed of two polypeptides, an α and β chain. Cleavage of C3 leads to the production of C3b and C3a¹.

C3a binds to C3a receptor, a G-protein coupled receptor which couples to G_i upon activation. Like all other members of the superfamily, C3a receptor has seven transmembrane domains with an extracellular N-terminus and intracellular C-terminal tail¹. The receptor has two glycosylation sites, an unusually large extracellular loop with a sulfated tyrosine residue important for ligand recognition and binding^2,3.

C3a receptor is expressed in cells of myeloid origin like neutrophils, macrophages and dendritic cells to name a few². It is also detected in astrocytes from inflamed brain⁴, endothelial cells⁵ and activated human T cells⁶. mRNA of C3a receptor is detected in lung, liver, kidney, brain heart, muscle and testis^2,7.