C1R also known as Complement C1Ris a member of the peptidase S1 protein family. C1R plays a vital role in the activation of the classical complement pathway. This pathway is essential for the immune response, as C1R helps to opsonize pathogens, promote inflammation, and facilitate the clearance of immune complexes. C1R exists as a dimer, which is activated through cleavage into two chains, A and B, allowing C1R to interact with C1Q and C1S to form the C1 complex. This complex is pivotal for initiating the complement cascade, leading to the generation of anaphylatoxins that enhance the inflammatory response. Understanding the structure and function of C1R is important, as dysregulation of the complement system can lead to various autoimmune diseases and inflammatory conditions.